Etiology, epidemiology and management of leaf and nut blight (Cryptosporiopsis sp) of cashew

Abstract

Leaf and nut blight of cashew caused by Cryptosporiopsis sp is a disease of economic importancein Tanzania. It causes significant yield losses of more than 48.4% on unsprayed cashew nuts. Cashew leaf and nut blight is severe especially after the off-season rainfall in coastal areas during the production phase. Keeping in view the economic importance of this disease, the present investigation emphasized upon identifying various isolates involved, cultural, morphological and physiological characteristics, developing a disease severity scale, developing management strategies and identifying sources of resistant to mitigate losses caused by the fungal disease. The isolates (AA1, AA2, AA3, AA4, AA5, AA6, AA7, AA8, AAA9 and AA10) from different cashew parts showed different percentage necrosis. Cryptosporiopsis sp isolatescaused chlorotic to irregular brown spots on host tissues. The conidiomata were acervular on leaveswith discrete conidiogenous cells and appeared cream colored. Conidiogenous cells solitary, cylindric, 9-12x12x2-5μm.The conidia were aseptate, hyaline, elongate ellipsoidal and brown to olive brown in color. Macroconidia averaged 40.6 ± 10.8 μm in length and 10.7 ± 2.3 μm in width, with a length/width ratio of 4.0 ± 1.4 μm.Paint brush method of inoculation was found to be more effective as compared to other two methods based on the lesion size.The direct transfer of conidia or conidiophores from the underside of the infected leaves to Petri dishes containing PDA using a sterile scalpel proved to be the quickest, easiest and most effective way to isolate the fungus. Colonies of Cryptosporiopsis spwere white initially becoming pale to dark brown in age. The Cryptosporiopsis sp isolates prefers pH range of 6.0-7.0. The fungus grew from 5-35°C, with optimum growth at 30°C and no growth above 40°C. Cryptosporiopsis sp fungi growth was enhanced by fluorescent light. A diagrammatic scale (0.1-3.0; 3.1-6.0; 6.1-12.0; 12.1-25.0; 25.1-50.0; 50.1-75.0%) with six levels was developed to assess cashew blight severity, caused by the fungus Cryptosporiopsis sp. The lower and upper limits of the scale proposed in this study were 0.2 and 67.4% of the infected area.Cashew clones AZA2 and AZA17 selected for the study were susceptible as the rate of infection (r) values were lower than other clones (AC10 and AC4) indicating that the spreading of thedisease was slow. AC4 and AC10 recorded higher area under disease progress curve (AUDPC) values and were considered as fast leaf blighters, while, AZA17 and AZA2 exhibited low AUDPC values and were identified as slow leaf blighters. Chlorothalonil 720g/L and Tebuconazole (4.5%) + sulfur (70%)showed 98% inhibition of mycelial growth while Mancozeb 80% (98%) and Mancozeb 480g/kg + Metalaxyl 100g/kg (96%) recorded the highest percentage inhibition of spore germination. Milicia excelsa and Corchorus olitorius L. extracts showed highest inhibitory effect against leaf and nut blight pathogen. FuPE (Penicillium sp), FuI (Aspergillus sp) and FuAS (Aspergillus sp) provided excellent potential antagonists capable of controlling the pathogenicity of Cryptosporiopsis sp on cashew trees.The following test species Manihot esculenta, Sorghum bicolor, Ipomoea batatas, cajanus cajan and Eucalpytuscamaldulensis,were infected by cashew leaf and nut blight pathogen when artificially inoculated suggesting they could be host plants. Saphrophytic survival capacity of the fungus Cryptosporiopsis spin cashew field revealed that the pathogen survival was 80 per cent up to four months of incubation but fell to 40 per cent after 6 months. In vitro evaluation revealed that, Chlorothalonil 720g/Land Mancozeb 80% were found effective. Among botanicals Milicia excelsa and Corchorus olitorius L. was effective. Among bioagents,Penicillium sp and Aspergillus sp respectively were most effective. Genotypes AZA2 and AZA17 were highly resistant under natural infection conditions.