Potato Seed Diseases
From Allotments4All
Coutesy of North Dakota State University
Contents
Virus Diseases
Certification readings are the best gauges of virus content. Virus symptoms are generally not visible in tubers, except for leaf roll, which causes net necrosis in certain cultivars, notably russets. Some varieties, notably Shepody and Russet Norkotah, do not readily express visible mosaic symptoms, and virus can be present without detection. Serological tests (ELISA) are often conducted for PVY mosaic in these varieties, either in the production or winter grow-out fields. Such tests are routinely used for PVX.
Rhizoctonia
The black scurf stage on the tubers should not exceed 5% of the tuber surface (Figure 1 – 47KB color photo). Coverage greater than this can result in yield and quality losses in the field. This disease is most active in cool soil and causes damage by pinching off the developing sprouts, which must regrow. This process delays emergence, lowers yield and increases off-grade tubers.
Fusarium Dry Rot
The seed should not have many tubers with symptoms of Fusarium dry rot (Figure 2 – 86KB color photo). A 1 to 2% level is reasonable and should not be exceeded in most years; federal regulations allow 1% dry rot at shipping. Fusarium inoculum on tubers without dry rot can be determined by cutting 20-30 tubers in half, placing them in a large paper bag or box, and shaking. Read dry rot after 10-14 days incubation at 50-60°F in high humidity. This will indicate the potential for Fusarium seed piece decay and the possible necessity for a seed treatment. In recent years, many isolates of Fusarium sambucinum (synonym = F. sulphureum), the agent responsible for dry rot, from throughout the US and Canada have become resistant to thiabendazole, the active ingredient in Mertect, resulting in reduced disease control. A modification of this test to determine resistance is to treat cut tubers with TBZ prior to incubating the seed in paper bags or boxes.
Bacterial Ring Rot
Look for external symptoms of cracks in the skin (Figure 3 – 59KB color photo); cut the stem end of suspect tubers for internal symptoms of ring rot. If no external symptoms are seen, cut the stem end of 100 or more tubers selected at random and look for characteristic ring rot discoloration in the vascular ring (Figure 3 – 91KB color photo). Because other diseases (Fusarium) or disorders (freeze injury) may resemble ring rot, squeeze suspect tubers and look for cloudy bacterial ooze from the vascular ring. Suspect tubers should be sent to a competent official for confirmation of ring rot. Do not use seed lots known to be infected with ring rot; seed lots with ring rot are not eligible for certification. The bacteria causing ring rot may be latent in a seed lot without causing symptoms for up to two years, and may be at such a low incidence as to avoid detection during visual inspections. Tests are available to index seed lots for the presence of latent ring rot bacteria. This testing can be done at the Seed Health Testing Lab at NDSU for a fee.
Soft Rot/Blackleg
If more than 1% of the tubers show symptoms of soft rot or tuber blackleg (Figure 4 – 61KB color photo), seed may have the potential for excessive seedpiece decay. Erwinia bacteria cause soft rot and wet seedpiece decay and can be latent. Because Erwinia can reside in the lenticels of the tuber, the number of tubers infested with (carrying) the bacterium gives an estimate of the potential for seed decay. This can be determined by wrapping 40-50 tubers in wet paper towels, and plastic wrap over that for 5-7 days at 50-60°F. A more severe test is to puncture 10 lenticels per tuber with a toothpick prior to wrapping. Soft rot should be odor free, mushy and wet but not sticky and stringy. A sticky, stringy decay with a bad odor indicates Clostridium decay and should not be scored. These tests will indicate the potential for soft rot seed piece decay if conditions are favorable for decay. Research results indicate that extra care is needed for seed lots with more than 50% tuber soft rot. If handled properly (see seed handling and planting recommendations), acceptable stands and yields can be obtained. Reducing bruising of seed during handling is the most important consideration for reducing soft rot in the seed.
Verticillium and Fusarium
These two fungal pathogens can cause wilt and early dying. They are easily visible as vascular discoloration in the stem end of the tuber (Figure 5 – 94KB color photo). Internally borne inoculum is not as important as inoculum on the surface of the tuber or in the soil. For the Red River Valley, the amount of Verticillium or Fusarium in the seed does not contribute greatly to the amount of wilt that results in the field. However, tuber borne inoculum does act as a source of Verticillium that will contaminate the soil.
Scab
This disease, caused by soil borne Streptomyces, is soil borne. Infected seed serves to introduce the scab organism into the field but does not provide inoculum for infected progeny tubers. Scab infection comes from scab in the soil, not the seed tubers. Excessively scabby seed is unattractive (Figure 6 – 54KB color photo).
Silver Scurf
This is primarily a seed-borne disease, although low levels of inoculum may survive in the soil from one season to the next. It is best to purchase seed with minimum amounts of visible scurf, but in reality, most seed lots have some silver scurf, and the disease may not be observed because many silver scurf lesions are too small to be seen. Silver scurf occurs as scattered golden/silver patches on the skin of the potato, primarily at the stem end (Figure 7 – 48KB color photo). The fungus causing silver scurf, Helminthosporium solani, sporulates on seedpieces shortly after planting and moves to progeny tubers during the growing season. Silver scurf-affected tubers are sources of inoculum in storage, and the disease spreads in storage, even at seed holding temperatures. Selection of disease-free seed is desirable, but impractical. The use of seed treatment fungicides is a better option for managing seed-borne inoculum.
==Late Bligh==t Presence of late blight in seed can be serious and lead to epidemics later in the season. Late blight can overwinter in infected seed and appears as irregular purplish lesions externally (Figure 8 – 33KB color photo), and, if cut about 1/4 inch deep, as granular rust colored areas internally (Figure 9 – 36KB color photo). Late blight tubers do not break down when stored cold (38F) and can act as one source of blight in the field when planted. Greater than 90% of late blight affected seed tubers decay due to secondary soft rot in the field at planting, but the few infected seed pieces that grow can start an epidemic under favorable conditions. Preliminary research has shown that late blight can be spread during cutting which can lead to infected plants early in the season. Seed treatment fungicides have been shown to reduce this infection. Federal regulations allow 1% late blight at shipping. If late blight is suspected, send a sample to a competent pathologist for confirmation. Late blight free seed is the best option.
Early Blight
This is a soil-borne disease caused by Alternaria solani and is sometimes found in seed lots. It is not considered a seed-borne disease, but a severely infected seed lot may suffer stand and emergence problems due to secondary soft rot and dead eyes.
Nematodes
Generally not a serious problem in the Red River Valley. However, certain areas may contain nematodes and more surveys are necessary to determine economic importance. Seed infected with nematodes should not be planted. Planting of nematode infested seed can be the initial source of field infestation which will increase in future years and be difficult to control.
Pink Rot and Leak
Most seed affected by these diseases, collectively called water rots, usually decays during the storage season and does not persist until spring for planting. However, affected seed should be removed at grading and not planted. Affected seed is watery and has a pink to charcoal black discoloration. Application of metalaxyl in the field when tubers are nickel to quarter in size provides excellent control of these diseases.
Planting
The three main points of planting are: 1) get good seed, 2) handle it carefully, and 3) use cultural practices that encourage quick emergence. The following checklist of cultural practices to follow at planting seed potatoes will minimize disease and maximize emergence and stand:
seed and soil should be the same temperature; 50°F is optimum avoid wet, soggy soils handle seed gently do everything possible to encourage quick emergence plant shallow and hill plants as they emerge
