Potato & Diseases - Part II:

Black Dot

    Research on black dot over the past few years has shed new light on many aspects of this fungal disease. Black dot is a difficult disease to control and there is no one silver bullet to knock it out

​Symptoms:The name "black dot" accurately describes the numerous dot-like, black sclerotia that can appear on tubers, stolons, roots, and stems both above and below ground level. A significant portion of the stem may be covered with sclerotia which are easily seen after vine kill. Total root growth is reduced and appears brown to black in color. Sclerotial bodies can be found on both roots and stolons. Some of the roots and stolons appear white in this photograph because the cortical tissue has been sloughed off due to severe infection. Apparently, enzymes secreted by the pathogen are responsible for the foot and root rot stages. Small, brownish lesions caused by Colletotrichum, bearing a resemblance to Rhizoctonia-induced lesions, may appear on recently infected stolons.

     Tuber infection appears as brownish to gray discoloration over a large portion of the tuber, or as roundish spots often larger than 1/4-inch in diameter. Silver scurf can also appear as patchy discoloration on the tuber surface, but takes on a silvery sheen when the affected area is moistened. Both pathogens frequently occur on the same tuber.

Control:

Seed: Purchase certified seed potatoes, since seed is the only way known to infest a clean field. There are no resistant potato cultivars, thin-skinned cultivars are more susceptible. Do not sterilize the seed.

Field: Use long rotations of 3-4 years. Keep the field free from debris, soil adequately fertilized, avoid excess water but keep the field well irrigated. Avoid skinning and bruising the tuber at the time of harvest. Note – there are no chemical control measures for black spot.

Control solanaceous weeds (ex: black nightshade)

Rotate with non-host crops (ex: tomatoes).

Powdery Scab

Symptoms: Tubers are typically raised pustules with a powdery mass. Spore balls, which are found in these lesions, can be seen with the aid of a microscope. Seeing the spore balls is the one sure way to identify the presence of powdery scab. Root galls will increase throughout the season. It a tuber has "pimples," set is aside for 24 hours. The color will change if the lesion is powdery scab.

    Symptoms can develop after harvest. Tuber to tuber spread in storage has never been documented. Contamination of seed pieces with spore balls is possible, however. Infected tubers may shrink in storage.

Control:

    There is no good control for powdery scab. Control measures need to both reduce the inoculum level in the soil and the inoculum level on the seed. Spores can be spread by contaminated seed tubers or by contaminated soil. Mancozeb-containing seed treatments may help reduce the spread of the pathogen from infected seed.

Gray Mold

     Gray mold originates primarily from infection of wounds such as punctures and bruises that are created at harvest and during the postharvest handling process. The decayed area appears light brown to dark brown and color is similar across the decayed area. The decayed area is spongy, and diseased tissue is not separable from the healthy tissue, which is different from blue mold (a soft decay)

Sclerotinia Stalk Rot (White Mold)

Control:

     Protectant fungicides may be applied to prevent gray mold developing on foliage

High levels of K and N are reported to reduce the percentage of tuber infection. Avoid mechanical damage during harvest.

If tubers are damaged during harvest operations, permit tubers to wound heal prior to low temperature storage.

Remove as much excess soil and stem material from potatoes during harvesting as possible.

Store potatoes at 40 degree Celcius.

For more information visit: http://www.seedbuzz.com/knowledge-center/article/potato-diseases-part-ii