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Seasonal Dynamics of Black Leaf Mould (Pseudocercospora fuligena) on Greenhouse‐Grown Fresh Market Tomatoes

Mersha, Zelalem, Zhang, Shouan, Hau, Bernhard
Phytopathologische Zeitschrift 2014 v.162 no.3 pp. 158-169
Pseudocercospora, correlation, disease severity, fresh market, fungicides, greenhouses, humid tropics, leaves, mancozeb, markets, models, pesticide application, relative humidity, tomatoes, Thailand
Black leaf mould (BLM), caused by Pseudocercospora fuligena, is a major fungal foliar disease of greenhouse‐grown tomatoes in the humid tropics. Quantifying the disease and yield loss from seasonal plantings will help mitigate the heavy reliance on frequent sprays of curative fungicides. In this study, severity of BLM was investigated in sequential plantings of tomatoes in two 50‐mesh BioNet greenhouses in central Thailand from June 2005 to January 2007. Each planting consisted of a block of six plants that received two applications of mancozeb and another block of six plants with no fungicide application at all. Two BLM peak epidemic periods were identified, that is, from plantings that were started in August–September and in January. On average, a yield loss of 30.6% was recorded from the two peak epidemic periods based on comparison of fungicide‐sprayed and non‐sprayed plants. Two sprays of mancozeb resulted in 71.6% reduction in disease severity during these peak epidemic periods. Mean disease severity (DS*) was highly correlated with a favourability index of relative humidity (RH), which was quantified on a scale of 0 (<85%, non‐favourable) to 1 (≥85%, extremely favourable). A three‐parameter logistic function explained the data well (R² = 0.81, P < 0.0001). Marketable yield (MY) was positively correlated with maximum plant height (PHₘₐₓ) but negatively correlated with DS*. In addition, MY was higher from plantings during October to December. It was predicted well (R² = 0.60; P < 0.0001) using the model MY = (a + b × PHₘₐₓ) × (1‐c × DS*), which combined both PHₘₐₓand DS*. Using this model, a reduction of 1.05% in marketable yield was predicted for each 1% increase in mean disease severity. The outcomes of this study implicated the need for management of RH and critical relevance of protecting tomato plants against BLM when they are grown during the peak epidemic periods.