The long wet spring has delayed potato planting by several weeks and many growers have already cut seed well in advance in anticipation of planting. This long period is ideal for incubation of seed-borne diseases and if seed has not already been treated with a fungicidal treatment, growers should consider an in-furrow application of an effective product.

Several diseases can cause problems at this stage and include bacterial soft rot for which only really cultural control practices such as aeration of seed piles and increased planting rates can compensate for; and fungal diseases the most important of which on plant establishment is Fusarium. Fusarium dry rot is one of the most important diseases of potato, affecting tubers in storage and seed pieces after planting. Fusarium dry rot of seed tubers can reduce crop establishment by killing developing potato sprouts. Growers should estimate the percentage of tubers affected (see symptom section below) and adjust planting rates as follows in the table. The extra percentage is to take in latent infection rates which for the sake of simplicity are doubled from the measured amount of infection.

Table 1. Planting rates of seed tubers based on percentage of tubers or seed-pieces with symptoms of Fusarium dry rot or other seed rots.

These recommendations are all based on estimations on what we have seen from experiments on Fusarium-affected seed in the field for other (related) purposes. We have not modeled latent infection, but from observation we do often observe in our experiments weakened plants that are less productive than non-infected plants. The recommendation therefore for growers is based on our best guess of the real percentage of plants that may be infected.  When infection rate is very high (greater than 15 percent measured), growers should consider the option of not planting the seed lot as transmission may result in higher contamination rate and increase risk of severe reduction in plant stand and further contamination of the field.

There is a further option of treatment of seed with a fungicidal seed treatment and, if not already done, a list of fungicides registered in Michigan for use on potato seed and in-furrow applications are shown in Table 2. 

Symptoms

The first symptoms of Fusarium dry rot are usually 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 usually dry (hence the name “dry rot”) and may develop at an injury such as a cut or bruise. The pathogen enters the tuber, often rotting out the center (Photo 1). 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).

Photo 1. Potato tuber infected with Fusarium dry rot (F. sambucinum), the cavity lined with yellow mycelium (left). On the right is a potato tuber infected with F. graminearum, lined with white to pink mycelium.

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 (Photo 2). Soft rot bacteria cause a wet, slimy rot, which can rapidly engross the entire tuber and mask the initial dry rot symptoms.

Photo 2. Dry rot lesion colonized by soft rot bacteria.

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. Infection of the sprouts and seedpiece decay may lead to uneven germination, hence a poor crop establishment (Photo 3).

Photo 3. Uneven stand of potato crop as a result of sprout infection and seedpiece decay caused by Fusarium spp.

Cultural control

Some level of Fusarium dry rot is almost always present in commercially available seed. Even though it is not possible at present to be 100 percent sure that a seed lot is completely free of dry rot, it is sensible to plant seed that meets established seed certification standards. Practicing the following procedures will help prevent dry rot:

• 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 minimize injury and promote rapid healing.
• Clean and disinfect seed storage facilities thoroughly 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 (see current recommendations for specific fungicides below).
• 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, plants 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.
• Monitor stored tubers often for dry rot. Grade out rotten tubers when tubers are removed from storage for marketing.

Chemical control

Seed treatment. Several products have been developed 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 (Colletotrichum coccodes). 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 (see previous article).

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 in the varieties Pike and FL1879. 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 and indeed recent studies at MSU have shown that greater than 80 percent of Fusarium species causing dry rot are sensitive to thiabendazole. 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.

Studies are under way at MSU to evaluate several of the new reduced-risk fungicides for use in postharvest applications. Some biological products have suppressed Fusarium dry rot in storage and include Serenade that is registered for foliar application to potatoes in the field.

Table 2. Seed treatments and in-furrow products labeled for use against seed and soil-borne early season diseases of potatoes in Michigan; extracted from MSU E312 bulletin 2011 (Click to view PDF).