The Quest to Identify a New Virus Disease of Sunflower from Nebraska.

Between 2010 and 2018, sunflower plants exhibiting virus-like symptoms, including stunting, mottling, and chlorotic ringspots on leaves, were observed from commercial fields and research plots from four sites within three distinct counties of western Nebraska (Box Butte, Kimball, and Scotts Bluff). Near identical symptoms from field samples were reproduced on seedlings mechanically in the greenhouse on multiple occasions, confirming the presence of a sap-transmissible virus from each site. Symptomatic greenhouse-inoculated plants from the 2010 and 2011 Box Butte samples tested negative for sunflower mosaic virus (SuMV), sunflower chlorotic mottle virus (SuCMoV), and all potyviruses in general by ELISA and RT-PCR. Similar viral-like symptoms were later observed on plants in a commercial sunflower field in Kimball County in 2014, and again from volunteers in research plots in Scotts Bluff County in 2018. Samples from both of these years were again successfully reproduced on seedlings in the greenhouse as before following mechanical transmissions. Symptom expression for all years began 12 to 14 days after inoculation as mild yellow spots followed by the formation of chlorotic ringspots from the mottled pattern. The culture from 2014 tested negatively for three groups of nepoviruses via RT-PCR, ruling this group out. However, transmission electron microscopy assays of greenhouse-infected plants from both 2014 and 2018 revealed the presence of distinct, polyhedral virus particles. With the use of high throughput sequencing and RT-PCR, it was confirmed that the infections from both years were caused by a new virus in the tombusvirus genus and was proposed to be called Sunflower ring spot mottle virus (SuRSMV). Although the major objective of this project was to identify the causal agent of the disease, it became evident that the diagnostic journey itself, with all the barriers encountered on the 10-year trek, was actually more important and impactful than identification.

Utilizing a Preplant Soil Test for Predicting and Estimating Root Rot Severity in Sugar Beet in the Central High Plains of the United States.

Aphanomyces cochlioides and Rhizoctonia solani are important soilborne pathogens causing root diseases that are primary constraints to sugar beet production in Nebraska, Colorado, and Wyoming. These types of diseases are difficult to control because they are often not noticed until substantial damage has already occurred. Efforts to manage them would be more effective if techniques were available that were more predictive than reactive. Therefore, a preplant soil test was developed to estimate the relative pathogen populations in the soil and to predict potential root disease problems later in the growing season. Preplant soil samples collected from fields to be sown with sugar beet were planted with a susceptible cultivar and tests were conducted for 1 month in the greenhouse. A preplant disease index was developed based on the time period during the test that seedlings became infected and was calculated on a 0-to-100 scale. Disease index values were compared with yields obtained from the same fields after harvest. Analysis of data collected for 5 years (2003 to 2007) with analysis of covariance revealed a strong relationship between the preplant disease index values and recoverable sucrose and root yields but not sucrose concentration. Results indicated that, for each unit increase in the preplant disease index, root yield decreased by 0.27 metric tons (270 kg) per hectare (P < 0.05, R2 = 0.44) and recoverable sucrose decreased by 49 kg/ha (P < 0.05, R2 = 0.45). We concluded that this preplant soil test can accurately predict root disease potential due to R. solani and A. cochlioides, and has the potential to help producers make effective management decisions in production fields using the index procedure. This soil assay has additionally provided new information on the biology, incidence, and distribution of root pathogens in production fields throughout the Central High Plains.

Characterization of management and environmental factors associated with regional variations in potato zebra chip occurrence.

Potato zebra chip (ZC), caused by the bacterial pathogen 'Candidatus Liberibacter solanacearum', which is vectored by the potato psyllid (Bactericera cockerelli), has caused widespread damage to U.S. potato production ever since its first discovery in south Texas in 2000. To determine the influence of environmental factors and management practices on ZC occurrence, data on management and meteorological variables, field locations, and psyllid counts were collected over a 3-year period (2010 to 2012) from six locations across the central United States (south Texas to Nebraska). At these locations, ZC-symptomatic plants were counted in 26 fields from systematically established 20 m × 30 m plots around the field edges and field interiors. Mean numbers of symptomatic plants per plot were classified into two intensity classes (ZC ≤ 3 or ZC > 3) and subjected to discriminant function and logistic regression analyses to determine which factors best distinguish between the two ZC intensity classes. Of all the variables, location, planting date, and maximum temperature were found to be the most important in distinguishing between ZC intensity classes. These variables correctly classified 88.5% of the fields into either of the two ZC-intensity classes. Logistic regression analysis of the individual variables showed that location accounted for 90% of the variations, followed by planting date (86%) and maximum temperature (70%). There was a low but significant (r = -0.44983, P = 0.0211) negative correlation between counts of psyllids testing positive for pathogen and latitudinal locations, indicating a south-to-north declining trend in counts of psyllids testing positive for the pathogen. A similar declining trend also was observed in ZC occurrence (r = -0.499, P = 0.0094).

First Evidence of a Binucleate Rhizoctonia as the Causal Agent of Dry Rot Canker of Sugar Beet in Nebraska.

Sugar beet (Beta vulgaris L.) is the primary source of domestic sucrose in the United States. In 2011, a sugar beet field in Morrill County, NE, was noted with wilting and yellowing symptoms suggestive of Rhizoctonia root and crown rot (RRCR), an important disease of sugar beet primarily caused by Rhizoctonia solani anastomosis group 2-2 (4). While the foliar symptoms were consistent with RRCR, the symptoms on the root were not. Root symptoms consisted of localized, dry, sunken lesions covering brown spongy tissue penetrating deeply into taproots. The surface tissues of the cankers distinctively produced a series of concentric circles. These root symptoms are inconsistent with RRCR, but are suggestive of a rarely occurring disease known as dry rot canker (DRC). DRC was first identified from Utah in 1921 (1), and assumed at the time to be caused by an uncharacterized strain of R. solani. It has since been sporadically but empirically noted from most western sugar beet growing states (4), but little is known about the pathogen or disease due to its infrequent appearances. To investigate the etiology of this disease, necrotic lesion borders were excised from diseased taproots, surface disinfested in 1% (v/v) sodium hypochlorite for 90 s, rinsed with distilled water for 90 s, and after drying on sterile tissue paper, placed on half-strength potato dextrose agar (½PDA) and incubated at 25 to 27°C. After 24 to 36 h, Rhizoctonia-like fungal growth was readily observed emerging from tissue pieces. Resulting colonies were tan to light brown. The ITS region of the rDNA was amplified from 4 isolates obtained from 4 distinct lesions and roots using the ITS1 and ITS4 primers (3) with standard PCR conditions, and sequenced (GenBank KC842197 to KC842200). The ITS regions were 100% identical between the 4 isolates and 96% (E-value = 0.0) identical to binucleate Rhizoctonia and Ceratobasidium sp. AG-F (e.g., JF519832, FR734295, JF705217). Hyphal cells were observed to be binucleate after staining 48-h-old cultures with lactophenol blue. Therefore, these isolates were identified to be a binucleate Rhizoctonia group AG-F based on morphological and molecular characteristics. Although distinct from DRC, a similar phenomenon has been recently reported from China implicating binucleate Rhizoctonia species with seedling disease in sugar beets (2). To determine pathogenicity of DRC isolates, 1- and 2-month-old sugar beet plants grown in 10 cm pots (5 plants per pot with 4 replications per isolate) were inoculated with all 4 isolates by placing 3 mycelial plugs (8 mm diameter) taken from the leading edge of ½PDA plates onto the soil surface of each pot. PDA plugs were utilized as controls. After ~3 weeks, root lesions resembling DRC were observed and isolates were recovered and identified from diseased plants as described above. No symptoms developed on control plants. To our knowledge, this is the first formally confirmed report of DRC on sugar beets in more than 75 years from the Western Hemisphere. The original investigator suspected that the isolates he found inducing this disease were different from typical R. solani isolates based on different symptoms (1). Our results, based on different symptoms but also with distinct molecular, biological, and pathogenicity traits, validate those suspicions while also fulfilling Koch's postulates with binucleate Rhizoctonia AG-F pathogenic to sugar beet that is distinct from the more common R. solani. References: (1) B. L. Richards. J. Agric. Res. 22:47, 1921. (2) P. P. Wang and X. H. Wu. Plant Dis. 96:1696, 2012.(3) T. J. White et al. Academic Press, San Diego, CA. 1990. (4) C. E. Windels et al. Rhizoctonia Root and Crown Rot. Page 33 in: Compendium of Beet Diseases and Pests. R. M. Harveson et al., eds. APS Press, St. Paul, MN, 2009.

Genetic diversity among Curtobacterium flaccumfaciens pv. flaccumfaciens populations in the American high plains.

Curtobacterium flaccumfaciens pv. flaccumfaciens is a Gram-positive bacterium and has reemerged as an incitant of bacterial wilt in common (dry, edible) beans in western Nebraska, eastern Colorado, and southeastern Wyoming. Curtobacterium flaccumfaciens pv. flaccumfaciens is diverse phenotypically and genotypically and is represented by several different pathogen color variants. The population structure of 67 strains collected between 1957 and 2009, including some isolated from alternate hosts, was determined with 3 molecular typing techniques: amplified fragment length polymorphism (AFLP), repetitive extragenic palindromic polymerase chain reaction (rep-PCR), and pulsed-field gel electrophoresis (PFGE). All 3 typing techniques showed a great degree of population heterogeneity, but they were not congruent in cluster analysis of the C. flaccumfaciens pv. flaccumfaciens populations. Cluster analysis of a composite data set (AFLP, PFGE, and rep-PCR) using averages from all experiments yielded 2 distinct groups: cluster A included strains with colonies of yellow, orange, and pink pigments, and cluster B had strains of only yellow pigment. Strains producing purple extracellular pigment were assigned to both clusters. Thus, C. flaccumfaciens pv. flaccumfaciens is diverse phenotypically and genotypically.

First Report of Ascochyta Blight Caused by QoI-Resistant Isolates of Ascochyta rabiei in Chickpea Fields of Nebraska and South Dakota.

Ascochyta blight, caused by Ascochyta rabiei, is a serious disease of chickpea (Cicer arietinum) and fungicide applications are used to manage the disease in the North Central plains (4). During the 2010 growing season, a commercial field near Stanley, SD was treated with pyraclostrobin (Headline, BASF, NC) and called a management failure by the grower. Similarly, limited efficacy of pyraclostrobin was observed in an ascochyta research trial near Scott's Bluff, NE. In both locations, symptoms and signs consistent with A. rabiei infection existed on leaves, stems, and pods; namely, circular brown lesions with concentric rings of dark brown pycnidia. Symptomatic samples were collected, disinfected with 95% ethanol for 1 min, rinsed with sterile water, placed in 0.5% NaOCl for 1 min, and rinsed again with sterile water for 1 min (4). Samples were air dried, placed on potato dextrose agar (PDA) plates for 3 to 7 days, and colonies with morphological characteristics typical of A. rabiei were single-spored and transferred to new PDA plates and incubated for 7 to 14 days. Three and six putative A. rabiei isolates were obtained from South Dakota and Nebraska samples, respectively. Morphological characteristics were consistent with A. rabiei; cultures were brown with concentric rings of dark, pear-shaped pycnidia with an ostiole, and conidia were hyaline, single-celled, and oval-shaped (2). Comparison of the internal transcribed spacer (ITS) region amplified from the genomic DNA of 3-day-old liquid cultures using ITS4/ITS5 primers by BLASTN searches using the nr database in GenBank (Accession Number FJ032643) also confirmed isolates to be A. rabiei. Mismatch amplification mutation assay (MAMA) PCR was used for detection of sensitive and resistant isolates to QoI fungicides (1). Confirmation of the presence of the G143A mutation was carried out by cloning an mRNA fragment of the cytochrome b gene using cDNA synthesized from total RNA of A. rabiei and CBF1/CBR2 (1,3). Total RNA was extracted from 3-day-old liquid cultures and it was used instead of genomic DNA for this PCR to avoid large intronic regions commonly present in mitochondrial genes. The G143A mutation has previously been correlated with resistance to QoI fungicides in other fungal plant pathogens (3). Also, these isolates were determined to be QoI-resistant in vitro by PDA amended with a discriminatory dose of 1 µg/ml of azoxystrobin (4). To our knowledge, this is the first report of QoIresistant A. rabiei isolates causing infections on chickpeas in South Dakota and Nebraska. QoI-resistant isolates were reported in North Dakota and Montana in 2005 and 2007, respectively (4). Of nearly 300 isolates collected from these states from 2005 and 2007, approximately 65% were determined to be QoI resistant (4). The widespread occurrence of QoIresistant isolates and reduction of fungicide performance in fields led the North Dakota State University Cooperative Extension Service to actively discourage the use of QoI fungicides on chickpeas in North Dakota and Montana (4). It is likely that similar recommendations will need to be adopted in South Dakota and Nebraska for profitable chickpea production. References: (1) J. A. Delgado, 2012 Ph.D. Diss. Department of Plant Pathology, North Dakota State University. (2) R. M. Harveson et al. 2011. Online. Plant Health Progress doi:10.1094/PHP-2011-0103-01-DG. (3) Z. Ma et al. Pestic. Biochem. Physiol. 77:66, 2003. (4) K. A. Wise et al. Plant Dis. 93:528, 2009.

New Outbreaks of Bacterial Wilt of Dry Bean in Nebraska Observed from Field Infections.

Bacterial wilt caused by Curtobacterium flaccumfaciens pv. flaccumfaciens was one of the more problematic diseases of dry bean (Phaseolus vulgaris L.) throughout the irrigated High Plains (Colorado, Nebraska, and Wyoming) in the 1960s and early 1970s, but has not been observed since that time. However, in August of 2003, plants exhibiting wilting and irregular, interveinal necrotic foliar lesions surrounded by a bright yellow border were found in three dry bean fields (market class Great Northern) in Scotts Bluff County, Nebraska. During 2004, plants exhibiting identical symptoms were additionally found occurring in more than 40 dry bean fields in western Nebraska. Affected fields were planted with dry bean from multiple market classes and seed sources, including yellow bean, Great Northern bean, and pinto bean, and incidence varied from trace levels to 80 to 90%. Isolations were made from leaf and stem tissues and seeds collected after harvest from infected plants, and all yielded slow-growing, creamy yellow or orange, fluidal colonies on nutrient broth-yeast extract medium. The bacterium was identified as C. flaccumfaciens pv. flaccumfaciens based on cell morphology (coryneformshaped motile rods), positive Gram stain and KOH reactions, fatty acid profiles, and BIOLOG (Hayward, CA) identifications. Great Northern (cv. Orion) plants were inoculated by bacterial suspensions (5 × 107 CFU/ml) injected into leaf axils adjacent to the first fully expanded trifoliolate and were incubated in the greenhouse under ambient conditions fluctuating between 24 and 35°C. Wilting symptoms developed 7 days after inoculation with foliar necrosis and yellowing symptoms appearing after 24 days. Identical bacterial colonies were reisolated from inoculated tissues, completing Koch's postulates. Although recent reports of wilt have been made in North Dakota (2) and western Canada (1) in 1995 and 2002, respectively, they were based only on the presence of discolored seeds observed in dockage from processing plants after harvest. To our knowledge, this report represents the first widespread observations of bacterial wilt from field infections in Nebraska in more than 30 years. References: (1) J. R. Venette et al. Plant Dis. 79:966, 1995. (2) T. F. Hsieh et al. Plant Dis: 86:1275, 2002.

Improving Root Health and Yield of Dry Beans in the Nebraska Panhandle with a New Technique for Reducing Soil Compaction.

A field study conducted during the 2001 and 2002 growing seasons investigated the integration of fungicide applications and tillage methods for reducing root health problems in dry bean (Phaseolus vulgaris) plants by alleviating soil compaction and its potential exacerbation of root disease. Several cultural practices were combined with applications of the strobilurin fungicide azoxystrobin. Soil compaction was created artificially throughout the entire plot area. Six treatments, consisting of four tillage treatments and two combinations of tillage or applications of azoxystrobin, were tested to alleviate the compaction and enhance root health. Tillage treatments included a compacted control with no additional tillage, formation of beds approximately 10 cm above soil surface, zone tillage with an implement using in-row shanks, and both zone tillage and bedding combined. Fungicide treatments utilized the combination of both zone tillage and bedding with fungicide applications, and a fungicide treatment singly. Effects of compaction on plant vigor and disease development and severity were evaluated 67 and 83 days after planting in 2001 and 2002, respectively, by a visual estimation of plot vigor and by destructively sampling and making root and hypocotyl disease ratings on dry bean plants from nonharvest rows. Soil resistance and moisture were measured in plots 80 and 104 days after planting in 2001 and 2002, respectively, to estimate degree of compaction. In both years, Fusarium root rot, caused by Fusarium solani f. sp. phaseoli, was determined to be the main root disease impacting plant health in studies. All measured variables (root disease index, plant vigor ratings, total seed yield, seed size, and soil resistance) were significantly improved by any treatment that included zone tillage prior to planting. No added advantages were observed for decreasing disease or improving root health and plant performance with the use of azoxystrobin or by planting on raised beds. This is the first study to evaluate zone tillage as a method of reducing plant stress and root disease in dry bean plants.

Widespread Occurrence of the Perfect Stage of Powdery Mildew Caused by Erysiphe polygoni on Sugar Beets in Nebraska.

Powdery mildew, caused by Erysiphe polygoni DC (synonym E. betae [Vanha] Weltzien), has been a sporadic and relatively minor problem for sugar beet (Beta vulgaris L.) growers in western Nebraska. Yield losses in this region have been limited, in part because of the use of effective fungicides, but also because infection occurs late enough in the season that treatment has often been unnecessary. The perfect stage had been reported only once in the United States until 2001-2002 when it was identified from Idaho and Colorado (1). The teleomorph was also noted from several fields in Scotts Bluff County in Nebraska in October 2002. The first appearance of the disease in 2003 occurred during the second week of August within five miles of the fields where the perfect stage was noted in 2002. On the basis of these observations, a survey was conducted between mid-August and mid-October to map the appearance and distribution of the perfect stage of E. polygoni within the Nebraska Panhandle growing region. During this time, between 45 and 50 fields were surveyed in six Nebraska counties. This represented the majority (70%) of the sugar beet acreage in Nebraska. The first finding of the perfect stage occurred in early September from multiple fields in the vicinity of and including the field where the asexual stage was first reported in August 2003. Ascomata measured 85 to 110 µm with one to four (mostly three) ascospores per ascus, resembling previous pathogen descriptions (2). Subsequently, every other field in the North Platte Valley where the oidial stage had been found also contained the perfect stage by the third week in September, including the Nebraska counties of Scotts Bluff (15 fields) and Morrill (7 fields). Outside the North Platte Valley, powdery mildew was not detected until mid-September and mid-October for the Northern Panhandle (Box Butte County) and Southern Panhandle (Kimball, Banner, and Cheyenne counties) growing areas, respectively. By October 1, the perfect stage was found in 9 of 10 fields exhibiting the disease in the North Panhandle, whereas the perfect stage was not found in the Southern Panhandle before harvest. Over 85% of surveyed fields infected with powdery mildew also harbored the perfect stage (31 of 36). Not only is the new and continued presence of the perfect stage potentially problematic for managing fungicide resistance and developing new cultivars with pathogen resistance (1), but it may also provide a means for overwintering in this area. This could result in earlier and more severe infections that would additionally require uncustomary treatment for powdery mildew control. The unusually early appearance of the disease and the high incidence of the perfect stage in Nebraska fields during 2003 further highlights these concerns and warrants closely monitoring future crops for continued epidemics. References: (1) J. J. Gallian and L. E. Hanson. Plant Dis. 87:200, 2003. (2) E. G. Ruppel. Powdery mildew. Pages 13-15 in: Compendium of Beet Diseases and Insects. E. D. Whitney and J. E. Duffus, eds. The American Phytopathological Society, St. Paul, MN, 1986.

Novel Use of a Pyrenomycetous Mycoparasite for Management of Fusarium Wilt of Watermelon.

Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum, is a destructive disease that limits watermelon production in many areas of the world. The discovery of several pyrenomycetous ascomycetes occurring naturally in association with different formae speciales of F. oxysporum identified these fungi as potential biological control organisms for watermelon wilt. One such mycoparasitic isolate, identified as Sphaerodes retispora var. retispora, was chosen for biological control and ecological trials in the greenhouse. Four different methods to inoculate the mycoparasite were evaluated, three of which utilized the parasite encapsulated into sodium alginate pellets. The other method employed root-dipping plants with mycoparasite ascospore suspensions. Ecological factors also were investigated, including the ability of S. retispora var. retispora to colonize watermelon roots, and its ability to survive in soil over time and reduce propagules of F. oxysporum f. sp. niveum. In the biological control studies, the use of the mycoparasite significantly reduced plant mortality and increased dry weights of watermelon plants after being challenged with F. oxysporum f. sp. niveum, compared with pathogen-inoculated controls. It appears that the incorporation of the parasite into alginate pellets in the planting mix at seeding may be the most practical method for future field evaluations of transplant-grown vegetable crops. In the ecological studies, the mycoparasite was recovered from infested soil after 9 months, but was only isolated from watermelon roots when applied in the presence of F. oxysporum. S. retispora var. retispora had no effect on F. oxysporum f. sp. niveum propagules after being applied to soils in the greenhouse.

A New Soilborne Disease of Dry, Edible Beans in Western Nebraska.

During July 2001, wilting, dry bean plants (Phaseolus vulgaris L.) at the 3- to 4-leaf stage were observed in a field near Ogallala, NE. Affected plants exhibited dark brown lesions on the hypocotyls and lower stems. Isolations from lesions on diseased plants on potato dextrose agar (PDA) yielded several isolates of an unknown fungus containing binucleate hyphal cells and numerous clamp connections. After growth on PDA for 6 weeks, all fungal isolates remained sterile and did not produce identifying spores, chlamydospores, or sclerotia. Two of the recovered field isolates were randomly selected for pathogenicity tests. Great Northern bean plants (cv. Beryl) were grown in a peat moss/perlite mix (3:1) in 10-cm plastic pots and inoculated separately with each of the two isolates at the two-true leaf stage. Two-week-old cultures grown on PDA (two 100 × 15 mm plates) were ground in 400 ml water in a blender for 3 to 4 min. Approximately 50 to 60 ml of the mycelial suspension was drenched around stems in each pot (two plants per pot), and incubated in growth chambers at 20, 30, and 35°C with a 12-h photoperiod. Six replicates were used for each isolate/temperature combination. After 4 weeks at both 30 and 35°C, fungi were consistently reisolated from stem lesions of wilting plants like those from the field-infected specimens. These fungi were also morphologically identical to the original isolations, thus confirming pathogenicity of the isolates and completing Koch's postulates. No disease was observed at 20°C. The pathogenicity evaluations were repeated twice with similar results. A group of fungi initially called sterile, white, basidiomycetes (SWBs) have been reported causing disease in other legumes from subtropical climates, including snap bean (2,4) and pigeon pea (3). Similar fungi isolated from bermudagrass were identified as Marasmius spp. after induction of sporocarps in vitro (1). The Nebraska isolates appear to be similar, if not identical to those from previous reports (1,2,3), but will be referred to as SWBs until a teleomorph has been identified. To my knowledge, this is the first report of these pathogens on dry, edible beans and on a legume crop from a temperate climate. References: (1) R. E. Baird et al. Plant Dis. 76:244, 1992. (2) C. M. Howard et al. Phytopathology 67:430, 1976. (3) W. J. Kaiser et al. Plant Dis. 71:1006, 1987. (4) D. R. Sumner et al. Plant Dis. Rep. 63:981, 1979.

The Influence of Irrigation Frequency and Cultivar Blends on the Severity of Multiple Root Diseases in Sugar Beets.

The effects of cultivar mixtures and two irrigation frequency treatments were evaluated over two seasons for their impact on a complex of sugar beet root diseases in three fields infested with the fungal pathogens Aphanomyces cochliodes, Fusarium oxysporum f. sp. radicis-betae, Rhizoctonia solani, and the viral pathogen Beet necrotic yellow vein virus (BNYVV). Irrigations after emergence consisted of two or five (two 1994 studies) and three or six (1995 study) applications of water for dry and wet treatments, respectively. Cultivar treatments included MH9155, HH67, Ranger, Rhizosen, and four combinations of these same cultivars. Disease progress was monitored through destructive sampling of plants exhibiting foliar symptoms typical of root disease during the season. At harvest, data on root and sucrose yields, sucrose percentage, and a root disease index were collected. No significant irrigation × cultivar treatment interactions were observed. Few significant differences were observed between irrigation treatments involving measured yield components. Reduced irrigations however, resulted in significantly lower disease incidence in all three repeated experiments when cultivar treatments were combined. No added benefits were observed for increasing yield or decreasing root disease by planting mixed cultivars, compared to the same cultivars planted individually. Several regionally adapted cultivars performed as well or better than mixtures under the unusually high levels of disease pressure in test fields. When few alternative options are available, sugar beet growers may still benefit from reducing irrigations, and growing locally adapted cultivars in soils severely infested with root pathogens.

A Severe Outbreak of Ascochyta Blight of Chickpeas in Western Nebraska.

Interest in chickpea (Cicer arietinum L.) production as an alternative crop to sugar beets and dry beans has rapidly increased in the Nebraska Panhandle. Production (concentrated near Alliance in Box Butte County) has increased from 80 ha in 1999 and 800 ha in 2000, to almost 4,000 ha in 2001. In mid-July 2001, symptoms in fields consisted of circular, dark brown lesions (0.5 to 1.0 cm) on leaves and pods containing pycnidia (225 to 240 µm) arranged in concentric rings. Symptom appearance followed unusually high rainfall events (over 25 cm the previous week), and the epidemic affected virtually all chickpeas planted in the area. Infected leaves incubated in humidity chambers in the laboratory produced nonseptate, hyaline, oval to oblong conidia measuring 8.5 to 10.0 × 4.0 to 4.5 µm that oozed out of pycnidia in a mucilaginous mass. Based on symptoms and fungal morphology (1), the pathogen was identified as Ascochyta rabiei (Pass.) Labr. Because of the seedborne nature of the disease, pathogen identity was further verified by planting seeds from infected pods in the greenhouse. Emerging plants exhibited similar symptoms to those from field infections, and the pathogen was identified from newly developed lesions. A section 18 emergency exemption was obtained for the use of the fungicide Quadris (azoxystrobin) on 2,400 to 2,800 ha in Nebraska. This measure averted serious losses, but 15 to 20% reductions in yield and quality were still recorded from treated fields. Infected plants collected from untreated fields had almost 60% losses in both pod number and total seed weight compared with uninfected plants. Disease occurrence in the Western Hemisphere was first reported from Canada in 1974, followed by other chickpea-growing states in the United States, including California, Washington, Idaho, and North Dakota (2-4). To my knowledge, this is the first report of the disease in Nebraska, and its presence poses a significant threat to the developing chickpea industry in the state, as the planted area is expected to increase in the foreseeable future. References: (1) D. J. Allen. The Pathology of Tropical Food Legumes. John Wiley and Sons, New York, 1983. (2) M. L. Derie et al. Plant Dis. 69:268, 1985. (3) P. Guzman et al. Plant Dis. 79:82, 1995. (4) W. J. Kaiser and F. J. Muelbauer. Phytopathology 74:1139, 1984.

First Report of Aphanomyces Root Rot of Sugar Beet in Nebraska and Wyoming.

Sugar beet (Beta vulgaris L.) plants exhibiting dull green and chlorotic foliage were first observed in a field near Dalton, NE, in late July 1999. Root symptoms included distal tip rot with internal, yellow-brown, water-soaked tissues. Isolations on MBV medium (1) consistently yielded Aphanomyces cochlioides Drechs. Water cultures produced primary zoospores that encysted at the tips of sporangiophores, followed by release of secondary zoospores within 12 h. Seedlings inoculated with zoospores began to die 2 weeks after emergence in a greenhouse. Symptoms on hypocotyls began as water-soaked lesions that turned black and thread-like. The causal agent was reisolated from infected seedlings, completing Koch's postulates. The disease was subsequently found in more than 15 separate fields, representing 5 of 11 sugar beet-growing counties in Nebraska and 1 county in Wyoming. In October, plants from the same fields were observed with stunted, distorted roots and superficial, scabby lesions associated with latent A. cochlioides infection. The pathogen could not be isolated from this stage but was confirmed by observing mature oospores within thin, stained sections under a microscope. The sections were additionally mixed with sterile potting soil and planted in the greenhouse with sugar beets. Several weeks after emergence, seedlings began to die, and the pathogen was reisolated. This represents the first report of Aphanomyces root rot and its spread in the Central High Plains. It also confirms that the described latent symptoms on sugar beet are caused by A. cochlioides. Reference: (1). W. F. Pfender et al. Plant Dis. 68:845, 1984.

Characterization of Fusarium Root Rot Isolates from Sugar Beet by Growth and Virulence at Different Temperatures and Irrigation Regimes.

Fusarium root rot, caused by Fusarium oxysporum f. sp. betae is one of several root diseases damaging to sugar beet production in Texas. As a primary symptom, most isolates produce a severe tip rot on the distal end of the taproot, in addition to discoloration of vascular elements and wilting. Tip rot distinguishes Fusarium root rot from another F. oxysporum f. sp. betae-incited disease, Fusarium yellows, which induces vascular discoloration and wilting but no root rot. This study was conducted to further characterize a selection of five Texas F. oxysporum f. sp. betae isolates representing different vegetative compatibility groupings, symptom types (tip rot, and non-tip rot), and hosts. Radial growth at six temperatures was measured for each isolate in culture on half-strength potato dextrose agar. Significant growth differences were detected, indicating a substantial amount of variation among the isolates. Virulence of isolates was evaluated by inoculating 6-week-old sugar beet plants with a microspore suspension and transplanting them into infested field soil. The plants were incubated at 20 and 30°C in controlled temperature boxes within the greenhouse and grown under two different irrigation schedules. After 6 weeks, plants were harvested and assigned a root disease rating, and root and foliar dry weights were determined. Disease ratings among isolates at 30°C resulted in three isolates (all tip rot isolates) being severe, one mild, and one intermediate. At 20°C, only one isolate caused appreciable root damage. Irrigation treatments had no effect on disease incidence or severity. Significant differences in colony diameter growth and virulence among isolates at the two temperatures provide further evidence of variation among Texas F. oxysporum f. sp. betae populations. Results also suggest that the tip rot phenotype may be induced by some genetic factor unique to tip rot isolates. Therefore, the form name F. oxysporum f. sp. radicis-betae is proposed for those isolates from Texas causing a tip rot.

Genetic Variation Among Fusarium oxysporum Isolates from Sugar Beet as Determined by Vegetative Compatibility.

One hundred sixty Fusarium oxysporum isolates were collected over a 3-year period (1992 to 1994) from diseased sugar beet and pigweed plants from seven counties in Texas. Disease symptoms on sugar beet included root-tip-rot symptoms with wilting and vascular necrosis, and wilting and vascular necrosis only. Pathogenicity testing on sugar beets indicated that 132 isolates of the 160 recovered were pathogenic and were considered to be F. oxysporum f. sp. betae (FOB). Of the 132 isolates of FOB, 28 were initially chosen as testers and paired in all possible combinations to estimate the number of vegetative compatibility groups (VCGs) present. Once VCGs were determined from the 28 isolates, a nitrate nonutilizing mutant (nit) 1 or nit 3 from each of the remaining isolates was paired against a Nit M from each of the established VCGs. Thirty-three isolates obtained from other sugar-beet-growing states also were tested for vegetative compatibility. A total of 95 of the 132 isolates of FOB from Texas were assigned to one of seven VCGs identified. Sixty-three isolates were assigned to VCG 1, with VCGs 2 through 7 containing 6, 16, 2, 2, 2, and 4 isolates, respectively. VCGs 1, 3, and 6 were recovered from both sugar beet and pigweed. Two additional isolates collected from Texas in 1987 also belonged to VCG 1. A number of the isolates collected from Texas could not be assigned to any of the seven established VCGs. These included two single-member Nit Ms, 11 self-incompatible isolates, and 24 of unknown affiliation. None of the isolates from any one state were compatible with those from any other state. Results suggest that substantial variation exists among sugar beet isolates of FOB from the U.S., and that these populations of F. oxysporum are apparently distinct and endemic to their respective areas.