Balsam apple belongs to the Momordica genus, which makes it a member of the squash family. Like most of the other cucurbits, the balsam apple is an annual running vine. It blooms delicate, yellow flowers on single stalks and produces an egg-shaped, bumpy fruit. Balsam apple plants grow well in USDA zones 9a, 9b, 10a, 10b and 11. Balsam apples are susceptible to various diseases.
Anthracnose is a balsam apple disease common in areas that receive frequent rain in warm seasons. A fungus (Colletotrichum lagenariumi) causes small, yellow spots to form. As they age, the spots turn a dark brown or a tan color. Severe anthracnose cases cause entire leaves to die. The anthracnose fungus can survive for up to five years in weeds of the cucurbit family or in the refuse from previous balsam apple crops. Anthracnose typically attacks balsam apple plants after the balsam apple fruits are well-formed.
The mosaic virus causes the plant's foliage to mottle, with patches of normal color intermingled with yellow. The mosaic virus symptoms run from mild to severe, depending on the environmental conditions. The mottling can be seen on both the affected balsam apple plant's leaves and fruit. Mosaic viruses are transmitted via insects, particularly striped cucumber beetles, melon aphids, green peach aphids and spotted cucumber beetles. The disease is occasionally transmitted via seed.
Downy mildew typically occurs during wet, cool weather. Affected leaves first turn yellow and mottled, which is often mistaken for nutritional deficiencies or foliar nematode damage. As the disease progresses, the downy mildew results in a soft and fluffy gray, purple or brown fungal growth forming on the underside of leaves. The downy mildew fungus (Pseudoperonospora cubensis) can survive only on live plant tissue and needs a film of water to germinate and infect foliage.
Powdery mildew typically starts as a circular, powdery white or gray spots on leaves, which is often mistaken for dust accumulation. As the disease progresses, these spots expand and coalesce, forming a continuous mat of mildew. This balsam apple disease stunts and distorts the infected plant's leaves, fruit and buds. It might also cause the invaded tissue to die and the leaves to drop prematurely. The powdery mildew fungus (Sphaerotheca pannosa) prefers high relative humidity, shady areas and densely growing plants.
In order to control common balsam apple diseases, only disease-free seeds should be planted. Balsam apple crops should be rotated with other cucurbits for at least three years. These plants should be planted away from surface run-off water since most of the balsam apple diseases need excess moisture to thrive. Infected balsam apple debris should be burned or plowed down after the harvesting season. Wild cucurbit weeds must be controlled. Balsam apple plants should not be handled when they are wet with rain or dew.