Alternaria Leaf Spot and Alternaria Leaf Blight are two related plant diseases that affect mirlitons (chayote). The fungus Alternaria cucumerina causes leaf blight, and the fungus Alternaria alternata f. sp. cucurbitae causes leaf spot.
Alternaria leaf spot.
The disease first appears as brown leaf spots with a yellow halo, and as the spots coalesce, they can turn the entire leaf brown (necrotic) and cause it to die. At that stage, it’s referred to as “leaf blight.
Leaf spot starts with concentric black rings that often have a yellow (chlorotic) halo around margins. Although the disease primarily affects leaves, it also causes brown lesions on the stems. Unlike anthracnose, alternaria does not penetrate the stems and block the flow of nutrients and moisture, so it won’t kill entire shoots and stems as does anthracnose. The vine will lose leaves, but usually survives the disease and produces in the fall.
There is no known fungicide that can be used to treat it if the temperatures exceed 88℉. Copper-based fungicides do prevent and eradicate the fungus, but they are phytotoxic in hot weather and can kill the plant. There are products like Daconil that coat the plant with a film, making it difficult for the fungus to establish itself on the vine. However, it must be applied from spring forward, and there’s no scientific study that indicates it’s effective as a preventative on mirlitons. Mirltion.Org is currently experimenting with a new biofungicide to test its effectiveness.
Treatment:
Do not water the vine from the top down–that will only spread the disease, as it also does with anthracnose. Remove all the dead leaves.
Because the disease primarily affects the leaves, the vine will continue to replace dead leaves with new ones. As a result, the vine will normally survive the summer and produce a crop in the fall.
Here is a link to several images of the disease on different crops.
Alternaria Leaf Spot. Early stage.
Advanced alternaria leaf blight. Soon, the entire leaf will become brown.