POTATO DISEASES

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.

Potatoes grow best in soils of pH 5.0 to 7.0. Deficiencies or toxicities of major or minor elements may be caused by excessive solubility or fixation in the soil through interaction with soil colloids or chemicals.

Nitrogen (N) requirements increase rapidly with potatoes plants growth.
When N is translocated to upper leaves excessively from the lower leaves, they then become yellow.

Later, if the deficiency is not corrected by fertilization, the entire potato plant becomes yellow and fails to grow properly.

Severity of plant response depends on the level of N deficiency. N toxicity from ammonium or nitrites may follow degradation of nitrogencontaining fertilizers in certain soil conditions.

Phosphorus (P) deficiency follows P fixation in a wide range of soil types.

Symptoms include retardation in growth of terminals; small, spindly, somewhat rigid plants with crinkled or cup-shaped leaves; darker than normal colour; possibly a delay in maturity; and reduced yield.

Potatoes tubers may have internal rusty brown necrotic flecks similar to internal heat necrosis.
Because P is frequently fixed in the soil, fertilizer banding applications lateral to the seed piece are superior to broadcasting.
Potassium (K) deficiency is common in light, easily leached soils.

Early symptoms are dark or bluish green glossy foliage.
Later, older leaves of potatoes plants become bronzed and necrotic (superficially resembling early blight), and senesce early.
Necrotic, somewhat sunken corky lesions form on the tuber surface, particularly at the stolon attachment.
Potatoes tubers are predisposed to black spot, and when cooked tend to darken.

Go Green with Propane Garden Power Tools at GreenGardenTools BEST PRICES on the Web.

(Phytophthora infestans)

Late blight is disease occurs while growing potatoes and caused by Fungi.
Although control measures exist, late blight remains the most serious fungal disease in most major potato production regions.

Symptoms of Late Blight

Water-soaked lesions appear on foliage that, within a few days, becomes necrotic, turning brown when dry or black when wet.

Under damp conditions, white mildew like sporulation is visible, especially on the lower surface of leaves. A pale yellow margin often forms around leaf lesions. Lesions on stems and petioles are black or brown.

Stem lesions are brittle and stems frequently break at the point of the lesion. Under certain conditions, wilting can occur on stems with lesions.
Disease is favoured by temperatures between 10 and 25°C, accompanied by heavy dew or rain.
POtatoes tubers infected by spores washed by rain from the leaves and stems into the soil have brownish surface discoloration.

Sections cut through tubers show brown, necrotic tissues not clearly differentiated from the healthy portions. Later secondary rot organisms develop in blighted tissue and rots spread in storage.

Management

Sources of inoculum are neighboring fields of potato or tomato, volunteer plants, and cull piles.

The last two sources can be removed. Soil survival occurs wherever the sexual stage (resting oospores) occurs as a result of the presence of both the A1 and A2 mating types, which can lead to early infections.

Once infection occurs in a field, control is a function of host resistance and spraying, mediated by the environment.

Potatoes growers should check with local extension agents for information on forecasting systems or spray schedules that maximize fungicide efficiency and resistance levels of cultivars. There are cultivars with various levels of resistance.
Both protective and systemic fungicides are available, but the latter should only be used according to regional or national strategies that have been developed to minimize the development of resistance in the pathogen.
To prevent tuber infection when growing potatoes, plants should be well hilled, foliage completely sprayed during the growth period, and vines permitted to mature and die naturally or be killed before harvest.

Presto Products GKL09515 Geobin Composting System