Characterization of Rhizoctonia-Like Fungi Isolated from Agronomic Crops and Turfgrasses in Mississippi.

Twenty-three isolates of Rhizoctonia spp. from agronomic crops and turfgrasses were characterized by cytological and pathological methods in order to establish the identity, pathogenicity, and virulence of Rhizoctonia spp. and anastomosis groups that occur on these hosts in Mississippi. Twelve isolates were identified as R. solani, including the five anastomosis groups (AGs) AG-1-IB, AG-2-2, AG-4, AG-5, and AG-13. Rhizoctonia zeae, R. oryzae, and eight binucleate Rhizoctonia sp., including R. cerealis, also were identified. R. solani AG-4 isolates were consistently the most virulent isolates on all hosts in pathogenicity evaluations. Pathogenicity of AG-2-2 and AG-5 isolates, binucleate Rhizoctonia spp., and R. oryzae varied between hosts. Two AG-2-2 isolates from bermudagrass or wheat were determined to be clonal isolates, with numerous self-anastomosis reactions. R. solani (AG-1-IB) was pathogenic on all graminaceous hosts. R. cerealis produced sharp eyespot symptoms on wheat and corn and minimal symptoms on cotton and soybean. This is a first report of R. cerealis as a pathogen of corn. R. zeae isolates were pathogenic on all hosts, including cotton and soybean. These results indicate that a diverse group of Rhizoctonia spp. occurs as pathogens on a wide variety of agronomic crops and turfgrasses in Mississippi.

Pod and seed mycoflora on transgenic and conventional soybean [Glycine max (L.) Merrill] cultivars in Mississippi.

A 2-year (1999-2000) study was conducted at Starkville and Stoneville, MS to determine if the occurrence of the mycoflora varied on Roundup Ready (transgenic) compared to conventional soybean (Glycine max) cultivars. A total of 7,658 fungal isolates were identified from the pod and seed tissues of four cultivars compared at growth stages R6 and R8. Ninety-nine percent of all fungi isolated were mitosporic fungi and ascomycetes. In both years, total fungal isolates from the two locations were greater from the pod (65%) than from seed (33%) tissues. Isolation frequency from conventional cultivars was 54% compared to 46% for the transgenic cultivars. The most common fungi identified that are reported pathogens of soybean included Alternaria, Cercospora, Cladosporium, Diaporthe, Fusarium and Verticillium spp. When main effects and interactions were compared among the frequency data for the fungal genera, significant differences occurred, but consistent trends were not noted. Isolation frequencies of Diaporthe spp. during the R6 growth stage, were significantly greater on the conventional than on the transgenic cultivars in both years of the study, but only at Starkville. Isolation frequencies from samples taken during the R8 growth stage were similar at both locations in 1999 and 2000. Fusarium spp. isolated at R6 and R8 growth stages from pod and seed tissues were significantly greater on conventional than on transgenic cultivars in 2000. Even though frequencies were often significantly different between the transgenic and conventional cultivars, the data was not consistent between locations, pod and seed tissues, or growth stages. The pod and seed mycoflora of transgenic and conventional soybean cultivars was, therefore, similar in Mississippi.

Rhizoctonia solani AG-13 Isolated from Corn in Mississippi.

Rhizoctonia solani K¨hn anastomosis group (AG) 13 was isolated from asymptomatic root tissue of a corn (Zea mays L.) seedling grown at the Black Belt Branch Experiment Station, Brooksville, MS. Rhizoctonia solani AG-13 was recently reported from cotton grown in Georgia (2). Rhizoctonia solani isolate MS-168 was successfully anastomosed with tester isolate AG-13 (courtesy of D. E. Carling, University of Alaska). The hyphal diameter at the point of anastomosis was reduced, the anastomosis point was obvious, and cell death of adjacent cells was observed. These results were confirmed by D. E. Carling and are consistent with C2 anastomosis hyphal reactions (1). Rhizoctonia solani isolate MS-168 was cultured on potato dextrose agar (PDA) and incubated at 27 ± °C with a 12-h photoperiod for 28 days. Mycelium was buff-brown to beige with diurnal zonation throughout the colony. Aggregates of bulbils developed in the center of the colony that were dark brown, dome shaped, and accompanied by brown exudate. Bulbils were submerged in the medium and scattered across the surface of the colony. The optimal growing temperature of MS-168 was 27°C. Two pathogenicity evaluations were conducted on 170 corn seedlings planted into soilless potting medium. Four-day-old corn seedlings were inoculated with 7-day-old PDA hyphal plugs (2 mm in diameter) of R. solani isolate MS-168 by placing the mycelium side of the hyphal plug in contact with the mesocotyl tissue beneath the soil surface. The hyphal plugs were covered with soil. The nontreated corn seedlings were inoculated with PDA plugs minus the fungus. Corn seedlings were maintained under environmentally controlled conditions at 27 ±2 °C with a 12-h photoperiod and watered to prevent wilting. Disease symptoms on mesocotyl tissue were rated from 1 to 4 in which 1 = no symptoms, 2 = a few pinpoint lesions and diffuse discolored areas, 3 = distinct necrotic lesions, and 4 = girdling lesions (3). Fourteen days postinoculation, treated seedlings had a significantly higher disease rating (1.5) than the nontreated control (1.0). Thirty of eighty-seven corn seedlings inoculated with MS-168 expressed symptoms of discoloration and pinpoint and necrotic lesions on the mesocotyl tissue at the site of inoculation. On the basis of the results of the pathogenicity evaluation, MS-168 can be characterized as weakly virulent on seedling corn when grown under controlled environmental conditions. The identification of R. solani isolate MS-168 (AG-13) from corn in Mississippi broadens the natural distribution of occurrence and host range of this anastomosis group. References: (1) D. E. Carling. Grouping in Rhizoctonia solani by hyphal anastomosis reactions. Pages 37-47 in: Rhizoctonia Species: Taxonomy, Molecular, Biological, Ecological, Pathology, and Disease Control. B. Sneh et al., eds. Kluwer Academic Publishers, The Netherlands, 1996. (2) D. E. Carling et al. Phytopathology 92:893, 2002. (3) C. S. Rothrock. Report of the cottonseed treatment committee for 1993. Page 216-217 in: Proc. Beltwide Cotton Conf. Natl. Cotton Counc. Am., Memphis, TN, 1994.

Fungal occurrence, disease incidence and severity, and yield of maize symptomatic for seedling disease in Mississippi.

A study was conducted in Mississippi from 1995 to 1997 comparing soil rhizosphere fungi isolated from Pioneer 3167 hybrid maize (Zea mays L.) planted on Brooksville silty clay and Memphis silt loam soils. Maize seedlings were collected over four sampling dates from conventional and no-tillage plots. Eleven fungal genera consisting of nineteen species were isolated from these plants; Trichoderma spp. were most frequently isolated, followed by Fusarium spp. The highest disease incidence occurred in tilled plots of the latest planting date on Brooksville silty clay when samples were collected 17 days after planting. Root disease was most severe in 1996 from seedlings planted on the last planting date in tilled plots sampled 17 days after planting. Yields were significantly (P < or = 0.05) higher on Brooksville silty clay soil than on Memphis silt loam in both 1995 and 1996. Yields were highest from no-tillage plots and from maize planted on the earliest date. There was a significant correlation between incidence of root infection and disease severity. There was no correlation between the incidence of root infection and yield or between disease severity and yield at either location.

Gaeumannomyces graminis var. graminis Isolated from Emerald Zoysiagrass in Texas.

Wilkinson and Kane (3) previously reported diseased zoysiagrass infected by Gaeumannomyces graminis (Sacc.) Arx & Olivier var. graminis in the spring in Illinois. Emerald zoysiagrass (Zoysia japonicum Steud. × Zoysia matrella (L.) Merr var. tenufolia (Willd. ex Thiele) established by sod in a home lawn for one year in Austin, TX, developed irregular, chlorotic, and, subsequently, necrotic patches 30 cm in diameter and larger in late summer of 1999. Patches were restricted to areas of the lawn receiving full sun. The lawn was fertilized, mowed at 2.5 cm, and watered daily during active growth. A thatch layer in excess of 1.9 cm was present. Tillers within diseased patches were removed easily from stolons. Crowns were rotted and colonized by dark brown septate hyphae (4.5 µm wide) and olivaceous brown lobed hyphopodia (25 × 21 µm). Diseased tillers were desiccated and newly developed leaves were chlorotic. Stolons were also chlorotic and developed water-soaked lesions adjacent to crowns. Diseased roots appeared light brown and brittle with strands of dark brown septate runner hyphae along the surface of the root axis and olivaceous brown growth cessation structures within the cortical tissue. Overall, symptoms were more severe on crowns and nodes than roots. A Gaeumannomyces fungus was isolated from root, sheath, and bud tissues. Taxonomy of the fungus was consistent with the description of G. graminis var. graminis by Walker (1,2). Diseased plants were washed free of soil and other debris and maintained in a moist chamber for 14 days. Perithecia were formed on leaf sheaths. Morphology of perithecia, asci, and ascospores was consistent with Walker's description of perithecia, asci, and ascospores of G. graminis var. graminis (2). Leaf buds and root tissue, colonized by G. graminis var. graminis, were plated directly onto potato-dextrose agar containing streptomycin sulfate and rifampicin (100 ppm, respectively). Colonies of sparse white, slightly aerial mycelium turning olive brown with age and producing lobed hyphopodia, developed from plated plant material. Hyphae at the margin of colonies curled back, characteristic of G. graminis var. graminis. Symptoms reported here are similar to those described by Wilkinson and Kane (3); however, the season and prevailing environmental conditions were different. References: (1) J. Walker. Trans. Br. Mycol. Soc. 58:427, 1972. (2) J. Walker. Mycotaxon 11:1, 1980. (3) H. T. Wilkinson and R. T. Kane. Plant Dis 77:100, 1993.

Take-All Root Rot of St. Augustinegrass: First Report in Mississippi.

Take-all root rot has been reported as a destructive disease of St. Augustinegrass home lawns in Florida and Alabama (1). In June 1998 and 1999, St. Augustinegrass home lawns in central Mississippi developed chlorotic, thinning patches ranging from 0.5 to 4.5 m in diameter. By August of each summer, plants within affected patches were necrotic and dead. Roots of affected St. Augustinegrass were necrotic and shorter than those of unaffected plants; nodes on stolons were necrotic, and lesions developed on internodes. Ectotrophic runner hyphae and dark brown, lobed hyphopodia were visible on roots and aboveground plant parts, respectively. Symptomatic tissues collected from St. Augustinegrass home lawns were plated onto potato dextrose agar (PDA); the incitant of take-all root rot, Gaeumannomyces graminis(Sacc.) Arx & Olivier var. graminis, was isolated. Verification of G. graminis var. graminis was based on colony morphology and taxonomic identification consistent with the description by Walker (2). G. graminis var. graminis isolated from symptomatic St. Augustinegrass was grown on sterile tall fescue seed and incorporated into sterile sand/peat moss mix. Asymptomatic St. Augustinegrass sprigs were washed, and roots were removed prior to planting in infested and noninfested soil. Plants were cultured in the greenhouse for 60 days. St. Augustinegrass planted into noninfested soil was asymptomatic while plants collected from G. graminis var. graminis-infested soil were symptomatic for take-all root rot. Crowns and roots of affected plants were necrotic; leaves were chlorotic and necrotic. Both runner hyphae and lobed hyphopodia were visible. G. graminis var. graminis was reisolated from symptomatic tissues and confirmed as the incitant of take-all root rot. This is the first report of take-all root rot of St. Augustinegrass in Mississippi. References: (1) M. Elliott. Plant Dis. 77:206, 1993. (2) J. Walker. Trans. Br. Mycol. Soc. 58:427, 1972.

Occurrence of Ergot (Sugary Disease) Incited by a Sphacelia sp. on Sorghum in Mississippi in 1997.

Ergot (sugary disease) incited by a Sphacelia sp. was observed on Sorghum bicolor (L.) Moench var. Sweet Leaf II at Leland and Pop-larville, MS, in October 1997. The disease was characterized by droplets of a thick, sticky, semi-transparent secretion similar to aphid honeydew. The caryopsis of infected florets was converted to relatively soft, chalky pink, tumescent, immature sclerotia (1.5 to 1.7 × 4 mm) with blunt ends. Abundant conidia (11 to 17 × 5 to 7 µm) were produced on stromatic tissue and in large numbers in the sugary secretion. This fungus is similar to that described by Zummo in West Africa (2). Velazquez-Valle et al. (1) recently reported the distribution of Claviceps africana Frederiksen, Mantle & de Milliano in the United States in 1997. The Sphacelia stage of the fungus found in Mississippi is similar to that reported from other areas of the U.S. Our report extends the range for this fungus, which is contiguous to those areas reported by Velazquez-Valle et al. (1). References: (1) R. Velazquez-Valle et al. 37th Annu. Mtg. Carib. Div., Am. Phytopa-thol. Soc., San Jose, Costa Rica, No. 77, 1997. (2) N. Zummo. 1984. Sorghum Diseases in West Africa. An Illustrated Text. USDA/USAID, Zaria, Nigeria.

Quantitative Relationship Between Leaf Rust and Wheat Yield in Mississippi.

Yield losses in wheat (Triticum aestivum) caused by leaf rust were evaluated in cultivar trials at five locations in Mississippi over a 4-year period from 1986 through 1989. Different levels of disease developed in the various trials over the 4-year period. There was no significant interaction between location and cultivar when yield data were collected from sites in the north and central areas of the state. A model derived from data for eight cultivars at two locations showed a negative linear relationship between yield and leaf rust. Total grain yield was reduced by 1% for each 1% increase in rust when the percentage flag leaf area covered by pustules was assessed visually at Feekes stage 11.1. Using this model, and rust ratings from three additional locations, predicted yields were statistically similar to recorded yields.

Selection for resistance to Cochliobolus sativus in tall fescue (Festuca arundinacea Schreb.).

Cochliobolus sativus (Ito and Kurib.) Drechsl. ex Dastur is a major foliar pathogen of tall fescue (Festuca arundinacea Schreb.) which can greatly reduce the quantity and quality of forages available for animal consumption. A greenhouse screening program was initiated to determine the inheritance of resistance to C. sativus in tall fescue over several cycles of mass selection. Resistance to C. sativus in four tall fescue cultivars was increased with 2-3 cycles of mass selection. Realized heritabilities were low to moderate (0.04 to 0.58) indicating that environmental influences on the expression of resistance are quite high. Variances were unchanged by selection, indicating that further improvement should be possible. However, progress with mass selection can be expected to be slow. Lesion size was decreased in each cultivar by selecting for lesion coverage. Lesion size, being independent of inoculum load and therefore less subject to environmental variation, should be considered as an additional selection criteria to improve the rate of progress.

Effects of Pratylenchus zeae and Quinisulcius acutus Alone and in Combination on Sorghum.

Host-parasite relationships of Pratylenchus zeae and Quinisulcius acutus, alone or in combination, were studied on sorghum in the greenhouse and laboratory. Q. acutus at 1,000 or 5,000 nematodes per 15-cm-d pot and P. zeae at 500 nematodes per pot significantly suppressed plant height and fresh and oven dry shoot and root weights. A mixture of 1,000 Q. acutus and 500 P. zeae per pot resulted in greatest suppression of growth. Roots of plants inoculated with Q. acutus alone were reduced in number and size and showed lesions and discoloration. Reproduction of this nematode 42 days after inoculation was much greater in treatments of 100 or 1,000 than 5,000 nematodes. The population density of the two species at 6 weeks after inoculation was significantly less when combined than for each species alone. When the two species were combined, reproduction of P. zeae was greater than that of Q. acutus, but the final populations per gram of root weight were the same. Q. acutus fed ectoparasitically on epidermal cells of sorghum roots in the zone of elongation and differentiation when observed under in vitro conditions.