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White Mold of potato

Tuesday, April 27th, 2010

White mold of potato( also called sclerotinia stem rot) is caused by the fungus Sclerotinia sclerotiorum (Lib.) de Bary.

The disease well develop in moist conditions and is especially common in fields with overhead-irrigation such as by means of a center pivot. Agricultural practices that promote extensive canopy growth and keep relative humidity and free moisture in the crop canopy for extended periods of time and reduce wind movement, favor disease development.

Symptoms of white mold

White mold symptoms first appear as water soaked lesions 14 to 20 days following row closure. Lesions usually first appear in the intersections between the stem and branches, or on branches and stems in contact with the soil. These become quickly covered with a white cottony growth that can spread rapidly to nearby stems and leaves if moisture is present for several hours. As lesions expand they can girdle stems causing foliage to wilt. White mold is also often accompanied by bacterial stem rot, especially under wet conditions.

When conditions become dry, lesions dry out and turn beige, tan or bleached white in color and papery in appearance. As infected tissue decays, hard irregularly shaped resting structures called sclerotia form on the inside and outside of decaying tissue. No stem rot symptoms are observed on below-ground tissues( i.e. roots, stolons or tubers).

Management of white mold disease

Effective management of white mold requires implementation of an integrated disease management approach. The disease can be controlled primarily through the use of cultural practices and foliar fungicides.

Good fertility management to prevent excessive canopy development will also suppress white mold. As such, cultivars that naturally produce thicker, dense canopies are at higher risk of white mold, than those that produce sparser canopies.

Use of the biological control agent Conithirium minitans, a parasite of S. sclerotiorum sclerotia, to reduce the sclerotia bank in the soil has yielded conflicting results between the regions where the experiments were conducted.

The most widely cultivated commercial growers of potato are equally susceptible to Sclerotinia stem rot. In the absence of resistant cultivars, chemical control with fungicides remains the most effective management tactic. Successful fungicide products include Iprodione (a.i. iprodione), Botran (a.i. dichloran), Omega (a.i. fluazinam), Quadris (a.i. azoxystrobin), Topsin (a.i. thiophanate-methyl) and Endura (a.i. boscalid).

The long-lived sclerotia can be killed by flooding for about 5 weeks. Rotations with nonsusceptible crops, including potato only every third year, along with removal and destruction of infected plants, help reduce this disease. Avoid
overhead irrigation.

Field, greenhouse and in-vitro experiments have shown that there are no significant differences in the effectivness of these compounds. Using of these fungicides at initial full bloom are effective in reducing the number of infected stems.
However, using of the same fungicides made at or prior to row closure following label recommendations were found to offer erratic protection at best.

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