Image Image Image Image

Apple Scab on Leaves

Apple Scab on Fruits

Apple Scab Conidia


Apple Scab Biology

Apple scab caused by the fungus Venturia inaequalis is one of the diseases with the highest economic importance worldwide. Apple scab occurs in every country worldwide in which apples are grown. V. inaequalis is a obligate parasite. This means it can only survive on green apple tissue. In times without vegetation it can only survive if it itself stays in dormancy. For this reason apple scab survives in the absence of green tissue in his fruit form (ascocarp, pseudothecium) on leaves on the ground. Beside of this there is evidence that can hibernate inside of dormant buds.

The pseudothecium is formed during summer on infected plant tissue. They are 90-160 µm in diameter and can be seen with the naked eye or with an hand lens. In spring meiosis and mitosis take place and the diploid pseudothecium is forming numerous asci containing 8 ascospores each. This ascospores are discharged in rain, and leaf wetness and they can infect the green tissue of the apple trees, if they come into a film of free water lasting long enough.

Ascospores and conidia are germinating in free water. The germination tube forms instantly an appressorium or it forms a longer germination tube with an appressorium at the end. It will form several appressoria in case age resistance of the host is blocking the penetration of the host tissue.

In the majority of the apple growing areas we will have leaf fall in autumn. Here apple scab epidemics will start in spring by the formation of ascospores in the pseudothecia. In areas where green leafs stay on the trees epidemic can start from conidia too. In a small extend this can also happen in areas with mild winters where the fungus can hibernate in the dormant buds. Ascospores formed during warm and moderate humid periods are discharged during rain or leaf wetness and moved to the green tissue of the trees by rain and wind (See Apple Scab Ascospore discharge model). Infection will take place if a water film is lasting long enough on the leaves (See Ascospore Infection Model).

Conidia will be formed if we have green infected tissue in the orchard. Conidia are formed in periods with higher relative humidity, what can occur nearly all nights. This Conidia are distributed by wind. They rest on the leaves or fruits and when leaf wetness will be present they are germinating and they will infect (See Apple scab Conidia Infection Model). To learn how to use plant disease models for apple scab control read Practical use of the Apple Scab Models.


(c) Dr. Heinrich Denzer, Pessl Instruments GmbH, Weiz, 2007


Created by system. Last Modification: Monday 19 of September, 2011 08:34:22 GMT by Lena.