The critical thing that you need to remember is that late blight needs living tissue to survive. When conditions are right such as the cold, wet weather and the thunder storms of 1993, late blight can survive very well and be moved as much as 50 miles in a day. For a disease to exist, it needs three things: a host, the pathogen and the right environment -- the disease triangle (Figure 1). Disease management guidelines aim at manipulating interactions of these factors, making disease development less likely. The most effective disease management programs will simultaneously use as many different approaches to control possible.
KEY 10 Recommendations to minimize late blight in potato fields:
1. Eliminate potato cull piles and all other sources of living tubers and eliminate volunteer potatoes from last season.
2. Be aware of the relative susceptibility to late blight of the potato varieties that you are planting. Russet Burbank and Snowden are moderately susceptible; Atlantic, Monona, Norchip, Red Norland, Russet Norkotah, and Yukon Gold are very susceptible.
3. Plant certified seed and be aware of the late blight situation in the field from which it was harvested. Check the "North American Certified Seed Potato Health Certificate" provided for each lot.
4. Minimize handling of seed tubers; if seed is cut, immediately treat with a mancozeb-containing fungicide.
5. Hill potatoes to ensure that young tubers are adequately covered by soil.
6. Fertilize and irrigate optimally for the variety.
7. Apply the first foliar fungicide treatment when recommended by disease forecasting models, confirmed sightings of disease or weather patterns favorable for carrying late blight from other states. Whichever occurs first.
8. Thoroughly scout each field throughout the season to detect late and/or early blight as soon as possible. Pay special attention to the first span of center-pivots, low-lying areas and along the sides of the pivot platform.
9. Kill vines totally prior to harvest because late blight does not sporulate on dead plant tissue. It's an obligate parasite.
10. Infected tubers should not be stored. Monitor storage carefully for any sign of tuber decay.
a. Warm seed to 50-55 F before cutting.
b. Cut and plant before sprout emergence.
c. Note: there is no seed treatment against late blight.
d. Plant in well-drained, moist soil.
e. Plant in soil at 50 F and rising.
f. Do not plant shallow.
g. Destroy unused seed-pieces or parts.
h. Do not mix seed lots.
i. Eliminate cull piles.
j. Form full hills/rows.
k. Monitor emerged plants in low, wet areas.
l. Apply a protectant right away if plants emerged and it rained.
m. Keep track of late blight severity values in your area (See "Forecasting").
n. Start treatment and manage applications intervals based on severity values.
o. Keep informed about possible late blight observed in your area.
p. Be informed about diseased tomatoes and peppers in neighbor gardens.
Disease development is most likely in shaded, cool, wet portions of the field. Fungicide should be immediately applied if there is indication of late blight anywhere in the area including in gardens as tomato and pepper are excellent hosts as well. The area under center-pivot irrigation that is most susceptible to late blight is the first 100 feet out from the pivot (center tower). This area comprises about one-third of an acre. In many surveys, late blight always appeared within the radius of the first tower when the field was infested, i.e., all fields with late blight had it in this area. Control of weeds such as nightshades which also are excellent hosts for the late blight pathogen need to be controlled in and around the field.
Late blight spores can be washed down into the soil through cracks or by splashing. When the spores get into the ground and reach the tuber zone, they can infect the tubers.
In the past, fungicides were not applied until late blight was visible. But, that is too late. A good fungicide program can greatly delay the development of the fungus and the onset of late blight (Figure 2). The fungicides have to be put on early. Applications made before row closure result in better coverage and are more effective than applications made when an extensive canopy is present. Protectants need to be applied before the late blight spores are on the leaf surface and germinate (Platt, 1994). When a spore is already present, the protectant will trap it on the leaf surface, and the germinated spore will enter the leaf eating it out. The protectants are great, but if late blight spores are present, we have to use the systemic fungicide.
Application of a fungicide requires a good coverage of the leaf. Fungicide applications made with ground rigs provide better coverage than those made by air. If chemigation is used, it is critical that it moves quickly and the least amount of water is used. It is not a preferred method; ground application then aerial is preferred over chemigation. Fungicides can be pushed easily right into the ground and not onto leaves. A tenth of an inch per acre water is about 2,700 gallons per acre and the potato plant can only hold 100 to 200 gallons per acre.
Fungicides are highly effective tools for late blight management when used properly. Fungicides available for late blight control have either systemic activity or protectant activity. Systemics move throughout actively growing plants and will protect plant parts not directly contacted by the fungicide, such as tubers and developing leaves. In contrast, protectant fungicides coat plant surfaces where they prevent or reduce the development of new infections.
For determining the time and frequency of applying fungicides can be estimated. A model (formula) was determined in the 1960s and refined a decade later and then computerized in the 1990s that predicts when environmental conditions are favorable for the development and spread of late blight. The model will be described in another panel (see "Severity Values").
DEAD vines are essential since the late blight fungus needs living tissue. Spores can fall off the vine to the ground and infect tubers while still in the ground. Tubers can be infected during harvest by contacting infected living vines. Diquat is recommended and the addition of copper sulfate would aid in the killing of spores. Don't use ammonium sulfate since that will cause regrowth. Flaming and sulfuric acid would kill the vines faster and will reduce exposed spores only. (The spores protected by soil or plant tissue are not expected to be seriously affected.) Note a faster vine kill will not cause the tubers to mature faster such as set skin. When desiccating a potato field, kill any nightshade even along the perimeter, road or fences.
Tubers have to be graded for late blight. Remember at the Seed Seminar in Rapid City, SD, late blight can pass from tuber to tuber in storage. Avoid bruising or wounding tubers since these are entry points for late blight as well as several other diseases. If late blight is suspected, cool storage to 38 F and keep relative humidity to 85%.
The risk of late blight can be greatly reduced by eliminating sources of inoculum. Presently, the late blight fungus is only known to overwinter within infected, living plant tissue. In potato production areas, infected tubers from the previous growing season are the most important overwintering host. Therefore, elimination of both cull piles and/or any other source of volunteer potatoes is one of the most important methods used for late blight control.
Cull piles are the principle source of late blight outbreaks in the following year (Kirk, 2003). Keep culls far away from the next year's potato fields. Remember late blight strains need living tissue to survive the winter. The best methods for dealing with cull piles are: Bury culls deep enough so that sprouts can not emerge. Chop the culls (discing over them is good) in the fall and let our cold winter freeze them which means also don't let the piles be too high and not freeze all the way through. Spreading is necessary so that heat is not built up within the cull pile and allow the microorganisms to survive. Making composite from cull piles is not the best method to handle culls. Late blight outbreaks have occurred near composited cull piles. The ideal would be to chop, freeze and bury.
- Kirk, W.W. 2003. Thermal properties of overwintered piles of cull potatoes. American Jounal Potato Research 80:145-149.
- Platt, H.W. 1994. Foliar applications of fungicides affects occurrence of potato tuber rots caused by four foliar pathogens. Canadian Journal of Plant Pathology 16:341-346.