Infection dynamics of Fusarium mangiferae, causal agent of mango malformation disease.

Conditions affecting germination and growth of Fusarium mangiferae, causal agent of mango malformation disease, were studied in vitro. Both conidial germination and colony growth required temperatures >5 degrees C and reached a peak at 28 and 25 degrees C, respectively. A minimum 2-h wetness period was required for conidial germination, reaching a peak after 8 h of wetness. High incidence of fungal colonization in buds, predominantly the apical buds, was detected compared with inoculated leaves. The pathogen was detected in the roots of inoculated soil 19 weeks postinoculation but not in aboveground parts of the plants, and symptoms of the disease were not observed, either. Dry, malformed inflorescence debris serving as a source of inoculum caused significantly higher colonization (52 and 20%) of inoculated buds, compared with that (0%) of the untreated controls. Incidence of sampled leaf disks bearing propagules of F. mangiferae from an infected orchard peaked in June and July and decreased during the following months, whereas airborne infections on 1-month-old branches was the highest in May and June, corresponding with inoculum availability released from infected inflorescences. Colonization pattern, determined in naturally infected vegetative and woody branches, was significantly higher in node sections than in the internode sections. This study sheds light on infection dynamics, colonization patters, and the disease cycle of F. mangiferae in mango.

Production of Didymella rabiei Pseudothecia and Dispersal of Ascospores in a Mediterranean Climate.

ABSTRACT Temperature and wetness conditions required for development and maturation of Didymella rabiei pseudothecia were determined in a series of experiments conducted in controlled-environmental conditions. Initial stages of pseudothecium formation occurred at temperatures ranging from 5 to 15 degrees C. Incubation at low temperatures was essential for subsequent pseudothecium maturation. This requirement was satisfied for chickpea stem segments incubated at 5 or 10 degrees C for three consecutive weeks or during periods of 3 or 5 days, separated by periods at higher temperatures. Following the low-temperature requirement, subsequent pseudothecium development was independent of temperature in the range tested (5 to 20 degrees C). Wetness was essential for pseudothecium production: pseudothecia formed and matured on stem segments maintained continuously wet but also on those exposed to periods of three or five wet days, separated by dry periods. The dispersal of D. rabiei ascospores was studied using chickpea plants as living traps in the field. Trap plants were infected mainly when exposed during rain but also in rainless periods. Results of this study enabled us to describe the developmental events leading to the production of the teleomorph stage and the dispersal of ascospores by D. rabiei in the Mediterranean climate of Israel.

First Report of Leaf Blight Caused by Alternaria longipes on Carrots in Israel.

Alternaria leaf blight, caused by Alternaria dauci (Köhn) Groves & Skolko, is one of the most devastating foliar pathogens of carrots (Daucus carota L.). Lesions appear as minute, necrotic, dark brown spots often initiated on the edge of the leaflet blade. They later enlarge in size and may merge into a large necrotic area, causing shriveling of the entire leaflet (1). In summer 2000, observations made in several carrot fields in the northwestern part of the Negev Region in Israel revealed infections that were atypical for A. dauci because they were initiated primarily in the middle section of the leaflet blade and were surrounded by a large yellowish area. A. longipes (Ellis & Everh.) E. Mason was consistently isolated from the lesions. Occasionally both A. longipes and A. dauci developed on the same leaves. The two pathogens differed in conidial morphology (size and shape of spore and beak) when cultured on potato dextrose agar medium. One hundred conidia of each species were measured. A. dauci conidia were 100 to 450 µm long and 6 to 15 µm wide, with a beak of up to 3 times the length of the conidium; A. longipes conidia were 35 to 110 µm long and 11 to 21 µm wide, and the beak measured one-third to one-half the length of the conidium. These measurements corresponded to the sizes listed previously (2). Inoculation of greenhouse-grown plants and completion of Koch's postulates confirmed that A. longipes is pathogenic to carrots. Conidia of both species germinated at temperatures from 5 to 36°C. In vitro tests revealed that A. longipes was less sensitive than A. dauci to fungicides commonly used in Israel in carrot fields. A fifty percent effective dose of chlorothalonil and difenoconazole was 3.0 and 0.2 µg a.i./ml, respectively, for mycelia growth of A. dauci, whereas the corresponding values for A. longipes were 10.5 and 3.0 µg a.i./ml, respectively. The prevalence of A. longipes in carrot fields and the influence of this pathogen on yields are currently not known. References: (1) I. Barash et al. Physiol. Plant Pathol. 19:7, 1981. (2) M. B. Ellis. Dematiaceous Hyphomycetes. CMI. Kew, Surrey, England, 1971.

Optimization of Chemical Suppression of Alternaria dauci, the Causal Agent of Alternaria Leaf Blight in Carrots.

Alternaria leaf blight, caused by Alternaria dauci, is a major constraint to carrot production in Israel. Israeli carrot growers apply prophylactic sprays at 3- to 10-day intervals throughout the season until harvest, up to 30 sprays in a growing season. In this study, we attempted to optimize the chemical suppression of the disease, in order to reduce fungicide use. The efficacy of nine fungicides was determined in two field experiments. All fungicides reduced disease severity, but there were significant differences in efficacy among them. The most effective were difenoconazole and chlorothalonil; less effective were copper hydroxide, tebuconazole, trifloxystrobin, and mancozeb; the least effective in our experiments were flutrifol, propineb, and iprodione. The effect of the time of spray initiation on fungicide efficacy was determined in three field experiments. Qualitative (analysis of variance) and quantitative (regression) analyses of the data revealed that initiating sprays after disease onset reduced control efficacy. Thus, an action threshold model could not be developed for A. dauci in carrots. The time before harvest at which sprays could be terminated was tested in two field experiments and it was found that terminating sprays 14 days before harvest did not significantly affect the overall control efficacy. The main conclusions derived from these experiments were tested and corroborated in two additional field experiments.

Rational Management of Didymella rabiei in Chickpea by Integration of Genotype Resistance and Postinfection Application of Fungicides.

ABSTRACT Various aspects of the integration of genotype resistance and chemical control of Ascochyta blight (caused by Didymella rabiei) in chickpea were examined in field experiments from 1993 to 1999 and in greenhouse experiments. Four commercially available chickpea cultivars representing a range of resistance to D. rabiei were used. The efficacy of chemical control in a highly susceptible cultivar was significantly (P < 0.01) related to the conduciveness of the environment to the pathogen. Adequate disease suppression (>80% control) was achieved when weather supported mild epidemics, but insufficient control (<20%) was achieved when weather supported severe epidemics. The contribution of genotype resistance to disease suppression in a moderately susceptible cultivar varied from <10% when weather supported severe epidemics to approximately 60% when weather supported mild epidemics. Spraying a moderately resistant cultivar resulted in 95% control when weather supported mild epidemics, but only 65% control was achieved when weather supported severe epidemics. The existing level of resistance in a moderately resistant cultivar resulted in 70% control when weather supported severe epidemics; fungicides improved control efficacy significantly to >95%. Under mild epidemics, moderate resistance alone provided >95% control. The level of genotype resistance available in a highly resistant cultivar was sufficient to suppress the disease under all weather conditions, even without application of fungicides. The possibility of relying on postinfection rather than prophylactic application of fungicides was tested in the greenhouse and in four field experiments. Activity of the systemic fungicide tebuconazole was detected when the fungicide was applied up to 3 days postinfection, and application of tebuconazole or difenoconazole in the field as a postinfection treatment (i.e., after rain or overhead irrigation) suppressed the disease as effectively as preventive applications and required fewer sprays. In two experiments, the interaction between genotype resistance and chemical control at various amounts of irrigation applied via overhead sprinklers (as a simulation of rain) was tested. The results show that both the level of genotype resistance and the quantity of water should be taken into account in deciding whether to apply a postinfection spray.