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GENOTYPE IDENTIFICATION OF PHYTOPHTHORA INFESTANS AND FORECASTING LATE BLIGHT IN MOROCCO
M. SEDEGUIl R.B. CARROLLl, A.L. MOREHART<1>, R. LAKHDAR<2>, A. ARIFI<2>, and A. EL ISMAILY<2>.
lDepartment of Plant and Soil Sciences, University of Delaware, Newark, USA; 2 Ministry of Agriculture(MAMVA), Rabat, Morocco.

Background and objectives
In Morocco late blight, caused by Phytophthora infestans(Mont.) debary, is the most economically important disease on potato and yield can be reduced by as much as 60%. The disease occurs between October and April which means that the fall, winter and spring crops are all exposed to this disease during part or all of the crop cycle. Field tomatoes are also affected and inoculum of P. infestans is ubiquitous in some areas. A potato late blight forecasting system, BLITECAST, was tested for possible adoption in Moroccan climatic conditions. In the absence of disease-free seed stock and resistant cultivars, disease management is based mainly on chemical control. The objective of this research was to reduce the number of fungicide applications needed for late blight control by predicting environmental conditions which favor pathogen development and identifying the fungus genotypes present in different potato and tomato growing areas of Morocco.


Material and Methods
Three cultivars ('Spunta', 'Desiree' and 'Nicoia') widely grown in Morocco were planted in replicated plots in northern and southern Morocco. The BLITECAST system computerized in the WISDOM program was implemented to verify its use under Moroccan weather conditions and to determine the threshold for local "severity value". Isolates of P. infestans collected from commercial potato and tomato fields in Morocco during three years of survey, were analyzed to determine pathogenicity on several potato and tomato cultivars, mating type, genotype at two allozyme loci [2], and relative sensitivity to metalaxy]. Response of isolates to metalaxyl was assayed by mycelial radial growth on metalaxyi-amended agar, by floating leaves inoculated with P. infestans on metalaxyl solution and via potato tuber disks deposited on filter paper saturated with metalaxyl solutions [1].


Results and conclusions
Results indicate that BLITECAST can be successfully adapted for use in Morocco by decreasing the "severity value" of 18 (commonly used in the United States) to 14 or 12. Statistical analysis of the data using GLM of SAS indicates that the cultivar 'Desiree' is more resistant to late blight than 'Nicoia' or 'Spunta'. The method of fungicide application also affected the severity of the disease. The most dominant isolates of P. infestans found in Morocco were pathogenic on the potato and tomato cuitivars tested, are Al mating type, have the same ailozyme pattern (Gpi lOO/100 and Pep 92/100) as US-6 genotype, and are highly resistant to metalaxyl with an EC50>250 g/l.


References
1. K.L. Deahl et al. Am. Potato J. 70:779-1993. 2. S.B. Goodwin et al. Plant Dis. 79:1181-1995.