Yam leaf spot

Guignardia dioscoreae. Previously known as Guignardia dioscoreae-bulbiferae and Phyllostricta dioscoreae. Guignaria is the sexual state, and Phyllosticta dioscoreae is the asexual state, i.e., spores are produced without mating strains combining.

Africa, Asia, North America, Oceania. It is recorded from Australia, American Samoa, Federated States of Micronesia, Fiji, Papua New Guinea, Samoa, Solomon Islands, and Tonga.

Yams, including Dioscora alata, Dioscorea bulbifera, Dioscorea esculenta, Dioscorea fasciculata, Dioscorea pentaphyla, and Dioscorea villosa. Commonly, the spots are seen on Dioscorea esculenta.

Leaf spots, circular or irregular, up to 10 mm diameter, tan or grey, merging, with a dark brown or black border (Photos 1-3). Both sexual and asexual fruitbodies are often present in the one spot; these can be seen as black pin points.

Spread of the disease occurs when the spores ooze from the fruitbodies during wet weather and are splashed by rain or carried in the wind.

Symptoms are most commonly seen on Dioscorea esculenta, but the damage is seldom serious enough for growers to seek control measures.

Look for the large, up to 10 mm wide, grey to tan leaf spots, with dark margins, on mature leaves. The spots often merge.

CULTURAL CONTROL
Probably the most important method of cultural control of this disease is to collect and burn or bury the vines after harvest, and practice crop rotation, where an interval of 2-3 years separates yams grown on the same land.

RESISTANT VARIETIES
The disease is most important on Dioscorea esculenta, Lesser or Chinese yam. In most countries where this yam is commonly grown there are several varieties. There is no report on differences in susceptibility on different varieties, but it is likely that some are more tolerant to infection, and tests would be worthwhile.

CHEMICAL CONTROL
There are no reports of attempts to control this disease with fungicides; however, should infections be so severe that fungicides were needed, use copper products, chlorothalonil or mancozeb.

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When using a pesticide, always wear protective clothing and follow the instructions on the product label, such as dosage, timing of application, and pre-harvest interval. Recommendations will vary with the crop and system of cultivation. Expert advice on the most appropriate pesticide to use should always be sought from local agricultural authorities.