Causal Agent: 

Fungi (Oidium sp.; Oidiopsis sp.)

 

Characteristic Symptoms:
 
Affected leaf consists of white powdery mold/pustules on the upper leaf surface (caused by Oidium sp.).
   
Yellowing of the leaf is observed in tomato leaves infected by Oidiopsis sp. White powdery mold is observed on the lower side of the leaf.
   
Conditions for Disease Development:
 
The disease is common in greenhouse-grown tomatoes and in damp, shaded areas.
   
Spores can germinate at any relative humidity when  temperature is cool (15-25°C).
   
Once infection has occurred, warm days (above 30°C) with cool (below 25°C) and humid nights favour rapid disease development. 
   
Spores are carried by wind to new hosts.
   
Disease incidence is greater using furrow or drip irrigation than using overhead irrigation. Spore germination is inhibited by water on plant surfaces for extended periods. 
   
Management and Control:
 
Maintain good field sanitation (i.e. remove & burn infected plants/plant parts as soon as symptoms are observed).
   
Avoid excessive use of nitrogenous fertilizer.
   
Increase light intensity/avoid dense planting or overcrowding.
   
Apply sulfur-based fungicide  (e.g. Kumulus®) or thiophanate-methyl (e.g. Tango, Topsin®) or azoxystrobin (Amistar®, MiradorⓇ, RobatoⓇ), difenoconazole (e.g. ScoreⓇ, MontanaⓇ, PursueⓇ, BashⓇ), tebuconazole (e.g. Folicur®).
   
References:
Compendium of Cucurbit Diseases (1996) by the American Phytopathological Society
   

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