Banana black cross; back cross disease, tar spot

Phyllachora musicola

Asia, Africa and Oceania. It is recorded from American Samoa, Australia, Fiji, New Caledonia, Niue, Papua New Guinea, Samoa, Solomon lslands, Tonga, and Vanuatu.

Banana and plantain varieties.

The spots are black, four-pointed stars, up to 60 mm long, most clearly seen on the lower surface of older leaves. The long axis of the star is parallel to the leaf veins, that is, at right angles to the length of the leaf (Photo 1). The spots are scattered, but sometimes occur in large groups (Photo 2).

A velvet-like mass of spores is produced on the lower surface of the spots. The spores are spread by rain and wind. This is the asexual state of the fungus. Sexual spores are also formed in the spots, and they spread the fungus too.

Usually, the disease is of minor importance. On susceptible varieties, it is worse when they are planted under shade. In this case, the spots are dense, covering most of the leaf surface (Photo 2). Further damage to the leaf occurs when the spots become infected by diamond leaf spot, caused by another fungus, Cordana musae (Photo 3). (See Fact Sheet no. 72 for symptoms of Cordana.)

Look for the characteristic black, four-pointed stars, on the lower surface of older leaves, with their longer axis parallel to the leaf veins. Use a microscope to see the spores on the underside of the spots. Look to see if the stars are associated with large diamond spots of Cordana.

CULTURAL CONTROL
Control measures are unlikely to be needed against this disease. If they are, priority should be given to reducing shade levels or planting the bananas in open ground.

RESISTANT VARIETIES
Cavendish varieties are resistant. The fungus usually attacks cooking and ladyfinger bananas. Some of the FHIA varieties bred in Honduras, Central America, and held by the Secretariat of the Pacific Community, are susceptible. In the lowlands of Papua New Guinea, varieties FHIA 02, FHAI 17, FHIA 18 and FHIA 23 were susceptible, with FHIA 02 moderately susceptible (class 3 on a scale of 1-5), and other FHIA varieties less so.

CHEMICAL CONTROL
The disease is of minor importance in commercial varieties and, even in those that are susceptible, it is unlikely to need control by fungicides; cultural controls and resistant varieties should be sufficient.