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Noni shot-hole disease (313)

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Photo 1. Leaf spots on noni caused by Guignardia morindae, reddish brown with a thin red border.

Photo 2. Reddish-brown spots of Guignardia morindae on noni leaf.

Photo 3. Guignardia morindae symptoms on noni leaves. Fungal fruiting bodies are visible within the spots, and the centres of some spots are falling out.

Photo 4. Spot falling out of the leaf blade to create the shot-hole effect of Guignardia morindae on noni.

Common Name

Noni shot-hole disease, noni frog-eye

Scientific Name

Guignardia morindae; this is the sexual state. and Phyllosticta morindae is the asexual state, i.e., spores are produced without mating types combining. Other names previously used are Phyllostictina morindae and Physalospora morindae.

Distribution

Narrow. Southeast Asia, Oceania. It is reported from Australia, American Samoa, Cook Islands, Federated States of Micronesia, Fiji, French Polynesia, Guam, Kiribati, Niue, Rotuma, Samoa, Tonga, Tuvalu, Vanuatu, and Wallis & Futuna.

Hosts

Noni, Morinda citrifolia, and other Morinda species.

Symptoms & Life Cycle

Frog-eye or shot-hole disease. Spots roughly circular, up to 2 cm diam., yellowish, grey or reddish brown with narrow red borders, and black fungal fruiting bodies sometimes visible within spot (Photos 1&2). The centre of the spots, where the fungus forms spores is transparent and often falls from the leaf to produce a 'shot-hole' effect (Photos 3&4).

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.

Impact

It is unlikely that the leaf spots will have an economic impact on noni by causing premature leaf fall and reducing fruit yields; however, there has been no research to check.

Detection & Inspection

Look for the near circular leaf spots with pale brown papery-thin centres, up to 2 cm diameter, completely or partly surrounded by yellow halos. Look to see centres of the spots fall out, responsible for one of the common names, "shot-hole". Black pin point fruit bodies can be seen in the spots.

Management

CULTURAL CONTROL
The lack of information on noni shot-hole disease makes it difficult to give recommendations on its control. There has been no research on it, so there is no evidence that the damage causes premature defoliation that affects yield, or if it does how much. Even if yields were reduced, it would be difficult to decide what control measures to implement: noni is a medium size tree grown in small plantations or in backyards so it would be difficult to apply cultural techniques, such of leaf removal or pruning to reduce spore infection.

Collecting the diseased leaves and burning them might be useful, but many of the spots fall out, and these contain the fruiting bodies, but would be too small to collect.

It is possible that wider than normal spacing would lower infection, especially as this might increase air movement around the trees and reduce the time that leaves are susceptible in spore germination and infection.

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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