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Epidemiology of Fusarium fruit and stem rot of greenhouse grown sweet pepper Smudja, Mirej

Abstract

Spore monitoring, using Burkard Continuous Recording Air Samplers, in Lower Mainland commercial pepper crops during three different growing seasons strongly suggests that ascospores of Nectria haematococca are both the primary and secondary inoculum. Fusarium solani spores were seldom detected and apparently play a minor role, if any, in the spread of Fusarium fruit and stem rot. No ascospores could be detected in monitored greenhouses upon completion of the end-of-season cleanup procedures indicating that detectable spore inoculum was not being carried over from one growing season to the next in commercial greenhouses. In "clean" greenhouses, which obtained seedlings produced by one propagator, high ascospore levels were detected with the introduction of the new crop. No symptoms were observed on the new seedling plants but perithecia of N. haematococca were observed on the surface of the rockwool cubes in which the seedlings were growing. Spores trapped from these perithecia were pathogenic to pepper seedlings and caused typical fruit rot in pathogenicity tests. Thus, rockwool cubes contaminated during propagation are one way of introducing the fungus into "clean" commercial greenhouses. Under controlled conditions of temperature and relative humidity (RH) ascospore discharge from moistened diseased fruit tissues was not inhibited by RH as low as 51% (the lowest tested). Ascospore discharge from air-dried diseased fruit tissue occurred only at RH >95%. Temperatures in the range of 15 to 30°C were not inhibitory for ascospore discharge, however, maximum discharge occurred at the lowest temperature tested (15°C). Ascospores discharged onto glass slides maintained at 55-60% RH (typical RH observed during the cleanup) survived from 6 to 18 days, depending upon temperature. Under "typical" greenhouse conditions ascospores remained viable on glass slides for up to 12 days. Ascospore germination occurred at all temperatures tested (15-30°C), but germination was greater and occurred more quickly at higher temperatures. Substantial ascospore germination occurred only during prolonged periods (8-12 h) of very high (>98%) relative humidity. Therefore, although perithecia of N. haematococca maybe present on the cubes from the early stage of the growing cycle, losses due to Fusarium fruit and stem rot will depend on the greenhouse climate management. Ascospore discharge from the rockwool cubes, which are constantly saturated with nutrient solution, can occur independent of ambient RH. Therefore, aggressive climate management, which avoids extended periods of high RH, apparently reduces disease by preventing ascospore germination rather then preventing ascospore discharge.

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