Observation on a predisposing factor in Curvalaria leaf spot disease of cashew seedlings in Nigeria
1996
Olunloyo, O.A. (Cocoa Research Institute, Tafo (Ghana))
A peculiar leaf spot disease has been consistently observed on cashew seedlings (Anacardium occidentale L.) regularly raised in the nurseries at the Cocoa Research Institute of Nigeria, Ibadan, Nigeria. The leaf spot, associated with Curvularia senegalensis (Speg.) subram., was preceded by interveinal chlorosis manifested on 8 week-old seedlings. This interveinal chlorosis was evident on young leaves while older leaves remained green. The points and margins of the affected leaves kept their green colour longest. The leaf spot infection symptom first appeared as a pin head on the chlorotic lamina and later enlarged with time. A 2 x 2 x 2 factorial experimental design involving combination of methods of inoculation and use of chlorotic and non-chlorotic plants was used. Inoculation of the leaves of cashew seedlings showing chlorotic and non-chlorotic conditions was carried out with 100,000 conidia/ml of Curvularia senegalensis. Only chlorotic leaves were infected and yielded Kock's Postulates. In water culture experiments, iron deficiency was found to be responsible for the chlorosis which predisposed 8-week-old cashew seedling leaves to infection by Curvularia senegalensis. This finding was strengthened by the failure of the fungus to infect normal green leaves and seedlings that received a high dosage of iron. Application of iron as chelate to the roots at 5cm soil depth at the rate of 50kg/ha corrected the deficiency 8 to 12 weeks after treatment and prevented reinfection. The nature of this deficiency in cashew seedlings is not clear but it is known that availability of iron can be reduced by high pH values and high phosphate. Optimum growth of cashew seedlings depends on a proper balance of nutrient elements in the affected soil.
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