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Causal Agent: 

Fungus (Oidium sp.)

Characteristic Symptoms:
 
Leaves, stems and petioles may be affected.
   
Affected part consists of white, talcum-like, powdery mold/pustules on the upper leaf surface.
   
Infected leaves turn yellow, wither and die.
   
Conditions for Disease Development:
 
The disease develops quickly under favorable conditions (dense plant growth and low light intensity).
   
High relative humidity is favourable for infection and conidia survival; however infection can occur at RH below 50%.
   
The optimum temperature for disease development is 20-27°C; Under field conditions, the disease stops at high temperatures (38°C and above).
   
The time between infection and symptom appearance is 3-7 days, in which large number of spores/conidia are produced.
   
Management and Control:
 
Use resistant varieties, if available.
   
Avoid planting in low, shady areas or increase light intensity in the area.
   
Provide good air circulation to minimize disease incidence and severity.
   
Avoid excessive use of N fertilizer or use a slow-release fertilizer.
   
Avoid furrow irrigation when infection is high.
   
Apply sulfur-based fungicide  (e.g. Kumulus®) or thiophanate-methyl (e.g. Tango, Topsin®) at the early sign of the disease.
   
When plants are susceptible, thorough application of fungicides such as azoxystrobin (Amistar®, MiradorⓇ, RobatoⓇ), difenoconazole (e.g. ScoreⓇ, MontanaⓇ, PursueⓇ, BashⓇ), tebuconazole (e.g. Folicur®) is necessary to minimize infection.
   
References:
Compendium of Cucurbit Diseases (1996) by the American Phytopathological Society
   

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