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Posts Tagged ‘Field Crops’

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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Minimization of risk Fusarium dry rot

Tuesday, April 27th, 2010

Fusarium dry rot is one of the most essential diseases of potato. It affects tubers in storage and seed pieces after planting. Fusarium dry rot of seed tubers can lessen crop establishment by killing developing potato sprouts.

Symptoms

The first symptoms of Fusarium dry rot are typically dark depressions on the surface of the tuber. In large lesions, the skin becomes wrinkled in concentric rings as the underlying dead tissue desiccates. Internal symptoms are characterized by necrotic areas shaded from light to dark chocolate brown or black.
This necrotic tissue is frequently dry (hence the name “dry rot”) and may increase at an injury such as a cut or bruise. The pathogen enters the tuber, often rotting out the center. Rotted cavities are often lined with mycelia and spores of various colors from yellow to white to pink depending on the species of the pathogen (several species of Fusarium cause dry rot).

Dry rot diagnosis may be complicated by the presence of other tuber pathogens. Soft rot bacteria
(Pectobacterium spp.) often colonize dry rot lesions, especially when tubers have been stored under conditions of high relative humidity or tuber surfaces are wet.

Soft rot bacteria cause a wet, slimy rot, which can rapidly engross the entire tuber and mask the initial dry rot symptoms. Dry rot also causes sprout death and when estimating the frequency of infected tubers growers should carefully examine the eyes (sprouts) to check if they are viable.

Measures to minimize contamination of Fusarium dry rot

Some level of Fusarium dry rot is almost constantly present in commercially available seed. Undergo the following procedures will help prevent dry rot:

  • Always plant only certified seed. It is critical to purchase seed with as little dry rot as possible, so always inspect seed carefully upon receipt.
  • After careful unloading, seed should be stored at 40° to 42°F and 85 to 90 percent relative humidity, and kept ventilated.
  • Warm seed tubers to at least 50°F before handling and cutting to reduce injury and promote rapid healing.
  • Sanitary and disinfect seed storage facilities systematically before receiving seed.
  • Disinfect seed cutting and handling equipment often, and make sure cutters are sharp to ensure a smooth cut that heals easily.
  • Do not store seed near a potential source of inoculum (e.g., cull piles).
  • Prior to seed treating (on conveyer to seed treatment hopper), grade out (remove) heavily infected tubers.
  • Treat cut seed with a seed treatment to control seed piece decay and sprout rot
  • Plant infected seed lots seed shallow (about 4”) in warm, well-drained soil to encourage rapid sprout growth and emergence, and lessen the chance for infection.
  • After emergence, plaints can be hilled to establish required bed depth.
  • In the fall, harvest tubers after their skins have set and when their core temperature is greater than 50°F.

Chemical control

Seed treatment
Several products have been produced specifically for control of seed borne potato diseases and offer broad spectrum control for Fusarium dry rot, Rhizoctonia, silver scurf and, to some extent, black dot These include Tops MZ, Maxim MZ (and other Maxim formulations + mancozeb) and Moncoat MZ.

The general impact of these seed treatments is marked by improved plant stand and crop vigor, but occasionally, application of seed treatments in combination with cold and wet soils can result in delayed emergence. The delay is generally transient, and the crop normally compensates.

The additional benefit of the inclusion of mancozeb is for prevention of seed-borne late blight.
Studies at MSU have shown that the most effective control of Fusarium dry rot is achieved by the application of an effective fungicide, such as fludioxinil (Maxim-based products), prior to planting.

Treatment of infected seed pieces with Maxim MZ (0.5 lb/cwt) at 10, 5 or 2 days before planting significantly reduced the percentage of diseased sprouts per tuber and significantly reduced seed piece decay.

Although it may not seem cost-effective to apply seed treatments to healthy seed, these results suggest that applying a seed treatment up to 10 days prior to planting can provide effective control of dry rot and increase rate of emergence, rate of canopy closure and final plant stand.

Postharvest fungicides
Mertect, thiabendazole remains registered for postharvest use on tubers. Few alternative compounds are available for potato tuber treatment in storage but include chlorine-based disinfectants such as sodium hypochlorite, calcium hypochlorite and chlorine dioxide. Limited information is available on the effectiveness of chlorine dioxide on potato storage pathogens, and results of some studies have suggested that chlorine dioxide does not provide effective tuber protection against Fusarium dry rot.

Some biological products have suppressed Fusarium dry rot in storage and include Serenade that is registered for foliar application to potatoes in the field.

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