Variability Among Forecast Models for the Apple Sooty Blotch/Flyspeck Disease Complex.

Several disease forecast models have been developed to guide treatment of the sooty blotch and flyspeck (SBFS) disease complex of apple. Generally, these empirical models are based on the accumulation of hours of leaf wetness (leaf wetness duration [LWD]) from a biofix at or near the phenological growth stage petal fall, when apple flower petals senesce and drop. The models recommend timing of the initial fungicide application targeting SBFS. However, there are significant differences among SBFS forecast models in terms of biofix and the length of LWD thresholds. A comparison of models using a single input data set generated recommendations for the first SBFS fungicide application that differed by up to 5 weeks. In an attempt to improve consistency among models, potential sources for differences were examined. Leaf wetness (LW) is a particularly variable parameter among models, depending on whether on-site or remote weather data were used, the types of sensors and their placement for on-site monitors, and the models used to estimate LW remotely. When SBFS models are applied in the field, recommended treatment thresholds do not always match the method of data acquisition, leading to potential failures. Horticultural factors, such as tree size, canopy density, and cultivar, and orchard site factors such as the distance to potential inoculum sources can impact risk of SBFS and should also be considered in forecast models. The number of fungal species identified as contributors to the SBFS disease complex has expanded tremendously in recent years. A lack of understanding of key epidemiological factors for different fungi in the complex, and which fungi represent the most challenging management problems, are obvious issues in the development of improved SBFS models. If SBFS forecast models are to be adopted, researchers will need to address these issues.

A New View of Sooty Blotch and Flyspeck.

Sooty blotch and flyspeck (SBFS) fungi colonize the surface wax layer of the fruit of apple, pear, persimmon, banana, orange, papaya, and several other cultivated tree and vine crops. In addition to colonizing cultivated fruit crops, SBFS fungi also grow on the surfaces of stems, twigs, leaves, and fruit of a wide range of wild plants. The disease occurs worldwide in regions with moist growing seasons. SBFS is regarded as a serious disease by fruit growers and plant pathologists because it can cause substantial economic damage. The smudges and stipples of SBFS often result in downgrading of fruit from premium fresh-market grade to processing use. This review describes the major shifts that have occurred during the past decade in understanding the genetic diversity of the SBFS complex, clarifying its biogeography and environmental biology, and developing improved management strategies.

Diversity and biogeography of sooty blotch and flyspeck fungi on apple in the eastern and midwestern United States.

Sooty blotch and flyspeck (SBFS) fungi on apple fruit were sampled from nine orchards in four midwestern U.S. states during 2000 and 30 orchards in 10 eastern U.S. states during 2005 in order to estimate taxonomic diversity and discern patterns of geographic distribution. Forty apple fruit per orchard were arbitrarily sampled and colonies of each mycelial phenotype were counted on each apple. Representative colonies were isolated, cultures were purified, and DNA was extracted. For representative isolates, the internal transcribed spacer (ITS) and large subunit (LSU) regions of ribosomal DNA were amplified and sequenced. In total, 60 SBFS putative species were identified based on ITS sequences and morphological characteristics; 30 of these were discovered in the 2005 survey. Modified Koch's postulates were fulfilled for all 60 species in an Iowa orchard; colonies resulting from inoculation of apple fruit were matched to the original isolates on the basis of mycelial type and ITS sequence. Parsimony analysis for LSU sequences from both surveys revealed that 58 putative SBFS species were members of the Dothideomycetes, 52 were members of the Capnodiales, and 36 were members of the Mycosphaerellaceae. The number of SBFS species per orchard varied from 2 to 15. Number of SBFS species and values of the Margalef and Shannon indexes were significantly (P < 0.05) lower in 21 orchards that had received conventional fungicide sprays during the fruit maturation period than in 14 unsprayed orchards. Several SBFS species, including Schizothyrium pomi, Peltaster fructicola, and Pseudocercosporella sp. RH1, were nearly ubiquitous, whereas other species, such as Stomiopeltis sp. RS5.2, Phialophora sessilis, and Geastrumia polystigmatis, were found only within restricted geographic regions. The results document that the SBFS complex is far more taxonomically diverse than previously recognized and provide strong evidence that SBFS species differ in geographic distribution. To achieve more efficient management of SBFS, it may be necessary to understand the environmental biology of key SBFS species in each geographic region.

Pierce's Disease of Grapevines: Identification of the Primary Vectors in North Carolina.

ABSTRACT In the past 10 years, the winegrape industry in the southeastern United States has experienced rapid growth; however, further expansion may be inhibited by Pierce's disease (PD). Epidemiological studies were conducted to identify the primary vectors of Xylella fastidiosa, the cause of PD of grape, by surveying sharpshooter population dynamics in the eastern Piedmont and Coastal Plain regions of North Carolina. Sharpshooter species were assessed for the presence of X. fastidiosa in the field. Leafhoppers were trapped in three vineyards in the eastern Piedmont and one vineyard in the northeastern Coastal Plain in 2004 and 2005. Four insects were identified as most abundant: Oncometopia orbona, Graphocephala versuta, Paraphlepsius irroratus, and Agalliota constricta. Adult specimens of O. orbona, G. versuta, and P. irroratus were tested for the presence of X. fastidiosa by nested polymerase chain reaction. In all, 27% of O. orbona, 28% of G. versuta, and 33% of P. irroratus trapped were positive for X. fastidiosa over the two seasons. Transmission experiments demonstrated that both O. orbona and G. versuta have the ability to transmit X. fastidiosa to grape. These vectors are likely to be important in all winegrowing regions of the Southeast, because their presence has been documented throughout the southern states. In DNA analyses, X. fastidiosa strains from insects trapped in North Carolina were genetically similar to one another and to the known "PD strain" from California. This is the first report of these two leafhopper species transmitting X. fastidiosa to grapevines in the Southeast.

Clarification of the Etiology of Glomerella Leaf Spot and Bitter Rot of Apple Caused by Colletotrichum spp. Based on Morphology and Genetic, Molecular, and Pathogenicity Tests.

ABSTRACT Morphological characteristics and vegetative compatibility groups (VCGs) of 486 isolates of Glomerella cingulata, Colletotrichum gloeosporioides, and C. acutatum collected from apple leaves with Glomerella leaf spot (GLS) symptoms and fruit with bitter rot symptoms in the United States and Brazil were studied. From this collection, 155 isolates of G. cingulata (93 from fruit, 61 from leaves, and 1 from buds), 42 isolates of C. gloeosporioides from fruit, and 14 isolates of C. acutatum (10 from fruit and 4 from leaves) were studied using mitochondrial (mt)DNA restriction fragment length polymorphism (RFLP) haplotypes. A subset of 24 isolates was studied by examining the sequence of a 200-bp intron of the glyceraldehyde 3-phosphate dehydrogenase (GDPH) nuclear gene. In addition, 98 isolates were tested for pathogenicity on leaves of cvs. Gala and Golden Delicious in the greenhouse, and 24 isolates were tested for pathogenicity on fruit of cv. Gala in growth chambers. In total, 238 and 225 isolates of G. cingulata were separated into four distinct morphological types and six VCGs, respectively. Five morphological types and six VCGs were identified among 74 and 36 isolates of C. gloeosporioides, respectively. Three morphological types and four VCGs were identified among 74 and 23 isolates of C. acutatum, respectively. Seven different mtDNA RFLP haplotypes were observed within isolates of G. cingulata, two within isolates of C. gloeosporioides, and two within isolates of C. acutatum. Phylogenetic trees, inferred based on maximum likelihood and maximum parsimony methods using the intron sequence, produced similar topologies. Each species was separated into distinct groups. All isolates tested were pathogenic on fruit, though only isolates with specific VCGs and haplotypes were pathogenic to leaves. Vegetative compatibility was a better tool than molecular characters for distinguishing isolates of G. cingulata pathogenic on both leaves and fruit from the ones pathogenic only on fruit. Isolates of G. cingulata capable of causing both GLS and bitter rot were included in haplotypes and groups based on the sequence analysis of the 200-bp intron that also included isolates capable of causing bitter rot only. Additionally, isolates of G. cingulata from the United States and Brazil which cause GLS were included in different haplotypes and sequence analysis groups. Therefore, one hypothesis is that isolates of G. cingulata from the United States capable of causing both GLS on foliage and bitter rot on fruit may have arisen independently of Brazilian isolates of G. cingulata capable of causing both GLS and bitter rot, and the two groups of isolates may represent distinct populations.

Population Diversity within Isolates of Colletotrichum spp. Causing Glomerella Leaf Spot and Bitter Rot of Apples in Three Orchards in North Carolina.

The population diversity within isolates of Glomerella cingulata and Colletotrichum spp. associated with Glomerella leaf spot and bitter rot of apples was studied in an orchard of cv. Granny Smith located in Wilkes County, NC, and one orchard each of cultivars Granny Smith and Gala located in Lincoln County, NC. Morphological characters and vegetative compatibility groups (VCGs) were used to determine diversity within the species. The relative frequencies of the morphological types found within each species in each orchard were also determined. G. cingulata was the predominant species associated with bitter rot in the three orchards and Glomerella leaf spot in the Gala orchard. In the three orchards, different morphological types were observed within isolates of G. cingulata and Colletotrichum acutatum, but not within isolates of Colletotrichum gloeosporioides. Isolates of C. gloeosporioides were not found in the orchard of cv. Granny Smith in Lincoln County. In the other two orchards, C. gloeosporioides represented the lowest proportion of the population. Three VCGs were found among isolates of G. cingulata (VCG-1, 2, and 6), two among isolates of C. gloeosporioides (VCG-9 and 10), and two among isolates of C. acutatum (VCG-15 and 16). VCGs 2, 6, 9, 10, 15, and 16 were found in the Granny Smith orchard in Wilkes County, VCGs 1, 2, and 6 in the Gala orchard in Lincoln County, and VCGs 2 and 6 in the Granny Smith orchard in Lincoln County. Differences in frequencies among the different morphological types found within the three orchards remained relatively similar throughout the season and from year to year, suggesting that the relative frequencies of G. cingulata, C. gloeosporioides, and C. acutatum remain stable in an orchard once the fungi are established.

Relationship Between Apple Fruit Epicuticular Wax and Growth of Peltaster fructicola and Leptodontidium elatius, Two Fungi that Cause Sooty Blotch Disease.

Sooty blotch severity varied among apple cultivars or selections surveyed in 1989 and 1992. No mycelial growth was observed on russetted areas of the cuticle that are considered impermeable. Ursolic acid and n-alkanes were the most prominent components of the epicuticular waxes of the cultivars or selections evaluated. Although there were differences in the relative proportions of these compounds among the cultivars, the differences were not related to the severity of sooty blotch. Peltaster fructicola and Leptodontidium elatius were grown on compounds that comprise the epicuticular wax of the fruit to determine if one or more of these were needed for growth. The fungi did not grow on any of the five major components of the epicuticular wax unless dilute apple juice was included. Scanning electron microscopy studies showed that mycelia of P. fructicola grew on the surface of the wax and did not appear to degrade it. Our studies support the hypothesis that P. fructicola and L. elatius fungi are epiphytes and obtain their nutrients not from components of the cuticle, but more likely from fruit leachates.

Development and Evaluation of a Standard Method for Screening for Resistance to Radopholus similis in Bananas.

The description and evaluation of a standard assay method for screening for resistance of bananas to the burrowing nematode (Radopholus similis) under greenhouse conditions is presented. Seven banana genotypes, ranging from susceptible to resistant, were used to evaluate the method. Banana plants from tissue culture, grown in 0.4-liter Styrofoam cups containing sterilized sand as substrate, were maintained in the greenhouse for 4 weeks before inoculation. Two hundred burrowing nematodes, reared in monoxenic carrot-disk culture, were used as inoculum for each container. Plants were kept in the greenhouse for an additional 8 weeks at about 27°C and 80% relative humidity after inoculation. Burrowing nematodes reproduced well in the susceptible cultivars False Horn, Grande Naine, Valery, and Lacatan, whereas the reproductive fitness was very low in the resistant cultivars Pisang Jari Buaya and Yangambi. An intermediate reaction between these two groups was observed with Pisang mas. A similar trend was obtained in a follow-up field test, which indicated that the method is accurate and reliable. Assessments of total-root necrosis associated with this pathogen were also comparable between greenhouse and field conditions. However, nematode effects on the roots were more severe in the greenhouse test than in the field. In spite of low nematode reproductive fitness, root necrosis was relatively high in the two resistant cultivars tested in the greenhouse trial.

Characterization of Benomyl Resistance in Mycosphaerella fijiensis, Cause of Black Sigatoka of Banana, in Costa Rica.

Sixty-eight and eighty-six percent of monoascosporic isolates of Mycosphaerella fijiensis from two banana plantations in Costa Rica, in which benomyl was used for ≈10 years to control black Sigatoka, were resistant to benomyl in February and November 1994, respectively. No resistance to benomyl was detected in isolates collected during February 1994 from farms with no history of benomyl use that were located ≈50 km from the nearest banana plantations. Only 1% of isolates was resistant to benomyl in a sample taken during November 1994. In three additional banana farms where benomyl had not been used for 3 to 5 years before sampling, ben-omyl resistance persisted at a high frequency. Benomyl-resistant and -sensitive isolates were distributed equally throughout the range of isolate sensitivity to propiconazole, indicating no relationship between resistance to benomyl and lower sensitivity to propiconazole but double resistance to these two compounds. Five benomyl-resistant and five benomyl-sensitive isolates of M. fijiensis were inoculated to banana plants under greenhouse conditions. Benomyl-resistant isolates were more aggressive than benomyl-sensitive isolates, as determined by measures of disease severity, incubation time, and number of lesions at 40 days after inoculation.

Reaction of Four Musa Genotypes at Three Temperatures to Isolates of Mycosphaerella fijiensis from Different Geographical Regions.

Two tetraploid banana hybrids, FHIA1 and FHIA2, with resistance to black Sigatoka, and two highly susceptible, naturally occurring triploids, Grand Naine and False Horn, were evaluated at three temperatures for their resistance to isolates of Mycosphaerella fijiensis from five geographical regions. The youngest open leaf of young plants was inoculated, and plants were incubated at 22, 26, and 30°C in growth chambers. Duration of the incubation period and disease severity were used to evaluate the reactions of the genotypes. The incubation period was the shortest at 26°C. Disease severity was greatest at 26°C on Grand Naine and False Horn, but there was no clear temperature effect for the FHIA genotypes. The incubation period was longer on both FHIA genotypes than on Grand Naine and False Horn. With few exceptions, isolates with the shortest incubation periods caused greater disease severity than those with longer incubation periods. The level of resistance between the two FHIA genotypes was similar, and both expressed high resistance across temperatures and isolates of M. fijiensis, indicating that no physiological races of the pathogen were detected. There were differences in durations of the incubation periods and disease severities associated with the geographical origin of the isolates. Isolates that originated in Honduras, Colombia, and Costa Rica produced more disease on Grand Naine and False Horn than did isolates from Cameroon and Asia. However, no differences associated with the geographical origin of the isolates were observed for both FHIA genotypes. Also, there were no differences in disease severities within isolates that originated from Honduras, Colombia, and Costa Rica.

Epidemiology and Control of Citrus Greasy Spot on Valencia Orange in the Humid Tropics of Costa Rica.

An epidemiological study of Mycosphaerella citri, the cause of greasy spot of citrus, was conducted for 2 years at Finca 6 and El Parque, Costa Rica. Ascospores were the primary source of inoculum; only a few conidia were trapped during the study. Ascospores were trapped first during late April or early May. The spore discharge pattern was associated closely with seasonal rainfall distribution; in both years, the number of ascospores trapped increased rapidly through May, peaked during early June, declined rapidly through July, and were negligible during the rest of the year. Most ascospores were trapped during a 6- to 8-week period. Three principal discharge patterns were observed. One pattern, associated with rainfall, was characterized by the release of large numbers of ascospores within 1 h of the beginning of rainfall. Ascospore release often continued for several hours after the rainfall ended. A second discharge pattern was associated with dew. Large numbers of spores were trapped during periods when dew was heaviest (0200 to 0700 h). A third pattern consisted of a combination of the first two patterns (i.e., discharge during evening rain followed by a second discharge associated with dew). No relationship was found between different combinations of rainfall data and either the first or peak ascospore catch. Similarly, there was no consistent relationship between weekly percentages of leaves at different stages of decomposition and ascospore catch that could be used to predict the period of peak ascospore discharge. Weekly total number of leaves on the orchard floor was a better predictor of total ascospore catch. Two sprays of a copper fungicide reduced greasy spot incidence, severity, and defoliation compared to an unsprayed control. Oil, sprayed twice as a standard grower treatment, did not reduce disease incidence or severity compared to the unsprayed control.