Gray Mold

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Grey mold is caused by the fungus Botrytis cinerea and is found wherever potatoes are grown in Idaho. The pathogen has a wide host range, causing disease on more than 200 different hosts. The disease is favored by very moist conditions and is especially common in fields with overhead-irrigation such as by means of a center pivot. It is considered an opportunistic pathogen of minor significance and is usually first found in the lower older, senescing leaves and also in plants that have been weakened by disease or adverse environmental conditions. Foliar lesions can be very similar to and are often confused with those caused early and late blight. Heavy infection will eventually lead to the early death of the plant which results in lower yields and tuber quality.

Fig 1a.  Foliar lesions develop in water droplets at the tips of leaves and are wedge-shaped and always surrounded by a chlorotic band of tissue. They may be confused with late blight lesions. Fig 1b.  On senescent leaves, lesions may be water-soaked and develop a slimy rot.


Foliar lesions develop during periods of cool, wet weather, occur at the tips of leaves and are wedge-shaped (Fig. 1). Lesions may be confused with those of early and late blight – they are often bounded by major veins and a chlorotic band of tissue (Fig 1). On senescent leaves, lesions may be water-soaked and develop a slimy rot (Fig. 2). Infection and colonization of leaves often leads to infection of petioles, which constricts the vascular system leading to the death of the whole leaf petiole (Fig. 3). Heavy infection can lead to upright stems with covered in dead leaves and petioles (Fig. 4). These symptoms can be similar to those of Verticillium wilt. When lesions occur under a dense crop canopy where humidity is high, a grayish black or brown mold is often visible on infected stems and leaves. Although very rare, at harvest a dry rot-like tuber infection can occur in storage. The tuber surface becomes wrinkled, and underlying tissue becomes soft, developing a watery, brown decay.

Fig 3.  Heavy grey mold infection of leaves leads to whole petiole death and defoliation. Fig 4.  Heavy grey mold infection can lead to upright stems with covered in dead leaves and petioles.  These symptoms can look very similar to those of Verticillium wilt.

Disease cycle

The pathogen overwinters on crop debris as sclerotia and mycelia. Pathogen spores are disseminated by wind, rain splash and irrigation, and infection requires high humidity and cool temperatures. The spores of this pathogen are ubiquitous as it is has a host range of over 200 other crops, many of which are grown in rotation with potato in Idaho. Spores collect in pooling drops of water in depressions on flowers, leaves and stems. In potato, infections often become established in flowers in midseason and later in senescing lower leaves which become stressed as a result of shading or excessive humidity.

Management and control

Effective management of grey mold requires an integrated disease management strategy similar to those used to control early blight and white mold. This disease is primarily controlled by the use of foliar fungicides. Most of the fungicides registered for use on potato in Idaho are protective against grey mold and are effective in preventing infection and disease development. However, they will not control disease once it has become established. Recent fungicide trials carried out at the University of Idaho, Aberdeen R&E Center have shown that the most effective timings for control of grey mold is to make the first fungicide application 7 – 14 days prior to row closure and then on a 14 day schedule after that. Fungicides containing pyrimethanil, such as Scala and Luna Tranquility were found to be most effective in controlling grey mold.