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Peach Scab Cladosporium carpophilum

Peach Scab, induced by the plant pathogenic fungus Cladosporium carpophilum, occurrs on peaches, nectarines, apricots and plums.

Symptoms

The disease affects twigs, leaves and fruits. The most serious damage results from fruit infections. Twig cankers begin as small, reddish lesions on current season's growth. These cankers expand slowly and may not be visible until mid summer. The small cankers have irregular margins, but do not cause sunken areas on the bark. Fruit lesions start as small greenish spots. These generally don't appear until the fruit is half grown even though infection occurred earlier in the season. Older lesions are approximately 1/4 inch in diameter and develop a dusty or velvety green appearance. The numerous lesions typically are clustered near the stem end of the fruit. Extensive spotting can result in fruit cracks, which serve as entrance points for several fruit rotting fungi.

Fungal pathogen

The fungus overwinters in lesions on twigs. Conidia are produced in the spring after petal fall and are windblown or splashed  by rain. The conditions which favor disease development are temperatures above 16°C for spore production, over 10°C (optimal 22°C to 27°C) for spore germination, and between 2°C and 35°C for disease development. Most infection occurs at the time, when the shuck split , although the fruit remains susceptible through harvest.

On the twigs the mycelium hibernates in the form of dark-brown spherical cells. It is possible that fungal conidia are able to overwinter. In springtime conidia are produced from resting spores or mycelium. Through rain and wind they will be carried to  the leaves and fruits. Germination follows shortly and the fungus penetrate into the plant tissue. Infection  starts  after the petals fall, but symptoms are often not seen for some weeks. Inoculations and infections continue until the fruit matures.

Infection of the fruit: Here spores and mycelium are attached closely to the surface between the hairs, forming a mat of short, plump cells. The fungus does not penetrate into the flesh of the peach, but the close contact of the fungus with the outer cells allows absorption of nutrition from the fruit through the unbroken walls. Evidently there is some injury to the outer cells.

The fungus is also able to  infect the twigs, developing lesions as previously described. In these diseased areas the pathogen is able to overwinter till the next saison.

Model :

Temperatures between 7 and 24°C with leaf wetness periodes from 19hours to 9 hours at 17°C to 19°C and increases to 17hours at temperatures of about 24°C (array with temperatues and leaf wetness hours for modelling) . if infection started a relative humidity of more hant 90% is enough to increase the infection. if rel. humidity is above 70% then stop the infection.

The fieldclimate Modell calculates in dependence of leaf wetness duration and temperature a risk model of Cladosporium carpohilum.