Aphids can also cause leaves to curl but if present, the insects or cast skins will be evident to the naked eye. The fungus overwinters on bud scales, on twigs and on fallen infected leaves. In spring the fungal spores are moved on to newly developing leaves by splashing water. During cool, wet weather the spores germinate on the young leaves and cause infection. Development of the disease slows as temperatures increase.
Older leaves are resistant to infection.
Consequently, the disease is most prevalent in the spring. Integrated Pest Management Strategies 1. Keep plant healthy but avoid excess fertilization. A healthy plant can better withstand the loss of leaves but excess fertilization can cause succulent tissue that is very vulnerable to infection. Clean up diseased leaves.