Some Aspects Of The Biology And Control Of Leaf And Fruit Spot Disease Of Citrus Caused By Phaeoramularia Angolensis Kirk In Kenya
Juma, Imelda M.A
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The leaf and fruit spot disease of Citrus also referred to as the Kitale syndrome, is a new disease to Kenya. The causal agent is identified as Phaeoramu7aria ango7ensis. The disease has assumed epidemic levels in Western Kenya and it is a matter of time for it to spread to all Citrus producing areas in the country. The control of the disease has been difficult and farmers have expressed fear that the fungus may be resistant to fungicides. The isolation of the fungus was reported to be difficult, and in culture the fungus was said to sporulate only in media containing an extract of Citrus leaf and fruit peel. The objectives of this study were to "isolate and characterise the causal agent of the leaf and fruit spot disease, determine the most favourable conditions for growth and sporulation of P. engoIenesis, and to carry out in vitro tests on the resp5nse of the fungus to selected fungicides. Field surveys have shown that the disease is severe on all species of Citrus growing in Trans-Nzoia district. It was found that the disease occurred in epidemics whose severity coincided with heavy rains from August to December. Random sampling indicated that 60.8% of a 11 fall en 1eaves and 100% of the dropped fruits were infected. On the average, leaf litter of a single tree was found to contain about 39.4 x 108 spores. Morphological characteristics of the fungus from naturally and artificially infected plants, and axenic cultures, showed that this fungus belongs to the genus Phaeoramularia. However, from the literature, the distinction between the genera Phaeoramu7aria and Phaeoisariopsis is difficult and requires a taxonomic reappraisal. Isolates of P. ango7ensis were tested for pathogenecity on disease free Citrus seedlings. An inoculum from two week old solid cultures produced typical disease s y m p tom son young Citrus plant s. Lesions developed slowly, taking almost seven months for definite necrotic lesions to be formed from the time of inoculation. Growth in liquid media was more than in solid media with a mean difference of 17% ± 8 The Orange leaf extract gave more growth than other media ( viz, Tap water, Potato dextrose cornmeal, Potato carrot), mean difference 25% ±. 15 Sparse sporulation was observed on all solid media amended with dextrose. No sporulation was observed in media without dextrose, liquid media and in solid cultures more than two weeks 01 Growth occurred between 6-30oC with the optimum at The fungus grew over a range of pH 3 to 8 with maximum growth occurring at initial pH of 5 to 6. The best mycelial growth was obtained in cultures incubated in continuous white light. In vitro tests with Copper Oxychloride (50%WP), Kocide 101 ( Copper Hydroxide, 50%WP) , Antracol (Propineb 70%WP) and Dithane M45 (Mancozeb 80%WP) showed that these fungic ides were effective in stopping spore germination. However, Kocide 101 and Copper Oxychloride were not effective against mycelial growth, but Antracol and Dithane M45 were. Their differential effects on mycelial growth was significant P<0.05, a 1though the difference between Kocide 101 and Copper Oxychloride, and between Di thane M45 and Antracol were not significant. The LD50 for Antracol and Dithane M45 was found to be less than 10ppm and that for Copper Oxychloride and Kocide 101 more than 3000 ppm.