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Diseases Caused by Phythlum spp. on Selected Crops Grown Under Hydroponic Culture System and their Control
Thesis Abstract:
Large scale hydroponic crop production is a recent technology in Malaysia, with Univers iti Pertanian Malaysia (UPM) playing a lead role in its research and development. Since its inception in 1982, pre- and post-emergent damping-off in cucumber, muskmelon, and tomato seedlings had been frequently observed. Isolation of Phythium spp. from roots of wilted mature cucumber, muskmelon, and tomato plants were done at UPM, Malaysia Agricultural Research and Development Institute (MARDI), and FIMA in Fraser's Hill. The mature muskmelon plants exhibited drastic interveinal chlorosis and yellowing of young and old leaves at the onset of fruiting. It was accompanied by general necrosis of the root system resulting in subsequent plant death. Isolation of Phythium spp. implicated them to be the causal agent. Since little is known about their role and control in the hydroponic system in Malaysia, studies were undertaken to characterize and identify the Phythium spp., establish their pathogenicity, determine the sources of inoculum, and investigate the in vitro and in vivo efficacy of some fungicides for their control. Through cultural and morphological studies, three Phythium spp., namely P. myriotylum, P. splendens, and a non-sporulating Phythium sp. were identified from six isolates obtained.
P. myriotylum was isolated from muskmelon, cucumber, and tomato grown in the hydroponic culture system at UPM in Serdang, and was characterized by its optimum temperature for growth at 35°C and its ability to grow at 40°C and above. It produced numerous clavate, knoblike, or sickle-shaped appresoria. Inflated, digitate sporangia that discharged zoospores were observed in grass-blade water culture. It formed abundant oogonia with less than I 0 diclinous antheridia in single cultures. Oospores were highly aplerotic with diameter ranging from 17.65 ± 0.72 to 21.15 ± 1.00 urn.
P. splendens was isolated from muskmelon at the FIMA hydroponic setup in Fraser's Hill, producing numerous large spherical terminal hypha! swellings in the culture which measured 40.0 ± 1.04 um in diameter. It did not form oogonia or oospores in single cultures. The optimum and maximum temperature for growth was 30°C and 35°C, respectively.
The unidentified Pythium isolated from the tomato plant grown in the UPM hydroponic system at the Genting Highlands produced slightly inflated, filamentous, dendroid hyphae which did not discharge zoospores. It neither formed hypha! swellings, appresoria, nor any sexual reproductive structures. Its optimum growth was at 30°C, maximum at 40°C. Protoplasmic streaming in young hyphae was observed.
P. myriotylum was the predominant Pythium sp. isolated at the UPM hydroponic system in Serdang. Pathogenecity studies showed that P. myriotylum caused pre-emergent damping-off of cucumber, muskmelon, and tomato, and post-emergent death of cucumber and muskmelon seedlings younger than seven days after sowing (DAS). Cucumber and muskmelon plants older than 7 DAS appeared to be tolerant to P. myriotylum infection.
P. splendens isolated from the FIMA hydroponic setup in Fraser's Hill and the unidentified Pythium sp. from the UPM hydroponic unit at the Genting Highlands were pathogenic in all three crops studied at the pre-emergent stage.
All isolates of Pythium viz P. myriotylum (4 isolates), P. splendens (1 isolate), and Pythium sp. (1 isolate) exhibited different pathogenic responses and isolates of the same species (P. myriotylum) varied in their pathogenic response. Pathogenecity studies further revealed that interveinal yellowing, root death, and subsequent plant mortality of mature muskmelon was not related to P. myriotylum infection but was rather a result of manganese and iron deficiencies.
The inoculum source and population studies showed that P. myriotylum was present in the hydroponic nutrient solution and roots of both mature, healthy and chlorotic muskmelon plants. Results also revealed that the population level fluctuations of Pythium were related to the phenology of the crop. P. myriotylum was also present throughout the whole nutrient solution in the system.
In vitro chemical efficacy studies indicated that the fungitoxicity varied according to Pythium sp. For P. myriotylum, etridiazole was the most toxic, followed by metalaxyl. Propamocarb hycrochloride, oxadixyl plus mancozeb, copper hydroxide, and phosetyi-AI ranked next in descending effectiveness. For P. splendens, metalaxyl was the most toxic, followed by etridiazole, oxadixyl plus mancozeb, copper hydroxide, propamocarb hydrochloride, and phosetyi-AI.
The four fungicides (copper hydroxide, metalaxyl, etridiazole, and propamocarb hycfrochloride) did not show any significant performance in the pre-emergent efficacy study. However, when applied into the nutrient solution as post-emergent treatment, metalaxyl was the most effective followed by copper hydroxide, etridiazole, and oxadixyl plus mancozeb, but these did not differ significantly in the control of the disease.