Corynespora Leaf Spot

This fungal infection is one of the more common in the southern states and more humid areas of the United States. It affects many types of plants, including flowering ornamentals and vegetable crops.

Host Plants

Carriers of Corynespora can be wide in variety. The fungus can infect the leaves, stems, and flowers of azaleas and other flowers, tomatoes, papaya, and other crops. It can also affect pumpkin, some types of melon, etc. It’s actually a relatively new fungus in the U.S., having only been identified in 1945, so it may infect other plants that have not been properly diagnosed.

Symptoms

The first manifestation will be lesions on the top surface of leaves. These are circular, reddish-purple spots that, with favorable conditions, will grow and spread rapidly. When they enlarge, the dead center becomes brown as the leaf dies. The infection can spread through leaves to stems and buds, fruits, and flowers.

On fruits, the lesions will appear more lightly colored and will generally spread more slowly.

Life Cycle

Corynespora, like any fungus, requires humidity and warmth to thrive. Spores can lie dormant through the winter if protected from direct frost (in mulches, top layers of soil, on seeds, etc). Once favorable conditions are met, the growth and spread of this fungus will happen quickly. In tropical climes, this blight can spread through a garden of tomatoes in less than a week.

How This Plant Disease Impacts the Plant

The leaves are badly damaged and often fall from the plant when the leaf spot kills them. Plant growth will be stunted and fruiting will be light and under-developed. Flowering plants will often have flowers destroyed quickly by the fungus.

Corynespora rarely kills the host plant, but usually severely stunts it.

Preventing Leaf Spot Disease

Prevention of any fungus is mostly about aeration and proper spacing. If spacing to allow air flow is given, most fungi will be kept at bay, including Leaf Spot.

Treating Leaf Fungus

Commercial options for treatment exist, including fungicides. These are generally effective on many plants infected with Corynespora. Other methods include “droughting” (leaving the plant in full sun with no water for 2-3 days), which has limited results.

Quarantine and destruction of infected plants is also an effective way to save a crop.

Resources:
http://mrec.ifas.ufl.edu/foliage/resrpts/rh_93_12.htm
http://www.freshfromflorida.com/pi/enpp/pathology/pathcirc/pp42.pdf

How to control corynespora on cucumbers.