Disease Management

by Barbara J. Christ and Sara May

A. Eliminate sources of disease carryover

The late blight fungus survives the winter in potato tubers. Practices that will help to reduce the amount of the fungus in the growing area include:

• Dispose of all cull potatoes including those originating from loading and unloading of storage, debris mixed in with soil from the storage area, and culls removed from seed lots prior to planting or cutting.
• Proper disposal includes complete freezing of cull tubers, complete consumption by livestock, burial at least three feet deep to prevent sprouting, or composting.
• Destroy volunteer plants.

B. Plant certified disease free seed

Make personal contact with the seed grower. Ask questions.

• There is a 1% tolerance allowed for late blight in certified seed. Symptoms are difficult to detect and isolation of the fungus is unreliable. Late blight is not at uniform levels across a production area; therefore, generalizations cannot be made concerning levels of blight that may be expected in seed.
• If there is concern that seed may have been exposed to late blight, there are seed treatments that will decrease the spread of the late blight pathogen and increase plant emergence and stand uniformity. See “Seed Treatments” on page 42 for Tops-MZ or Evolve.
• When blighted seed pieces are planted, most will immediately rot and not sprout. However, when contaminated seed pieces (exposed to the pathogen, but not showing symptoms) are planted some will germinate, but the sprouts will be killed before emergence, some will emerge with late blight infections, and some will escape disease entirely.

C. Scout fields and be aware of late blight in nearby areas

• Growers should scout fields twice a week or more, especially if there have been several days of fog or rainy weather. Check areas where dew periods are prolonged by natural obstructions, such as overhanging tree lines, which can induce ideal conditions for infection. Check areas where it is difficult to get adequate coverage when applying fungicides. Also, check the early maturing varieties first and more frequently.
• Symptoms of Late Blight: Irregular, blackish-brown lesions on stems or leaves. When the relative humidity is high, there should be white mold on these lesions especially on the lower leaf surface or on stems. Be sure to check within the plant canopy on the lower stems. See “Identifying Potato Diseases in Pennsylvania” for sale through the Penn State College of Agricultural Sciences Publication Distribution Center, 112 Agricultural Administration Building, University Park, PA 16802 or call 814-865-6713. Information can also be found at our potato health website, http://potatohealth.cas.psu.edu.

D. Monitor your weather

Stations monitoring the weather are limited; therefore you must use common sense to be aware of your own weather conditions. Ideal conditions for blight are: daytime temperature between 60-75ºF and nighttime temperature between 50-60ºF, accompanied by fog, heavy dews or rain over a four to five day period. Extended periods of high relative humidity are also important for this disease.

E. Cultural practices:

• Rotate. Planting potatoes in fields as far from last year’s potato fields as possible will provide a distance between your crop and volunteer potatoes. Also, we have established that you can reduce early blight severity by increasing the distance between fields, even short distances.
• Hill plants. Proper hilling will promote healthier plants and reduce late blight infections on tubers.
• Avoid excessive nitrogen. Excessive vine growth can promote conditions for late blight. However, proper nitrogen levels will reduce early blight susceptibility.
• Vine kill. Thoroughly killing vines and keeping fungicides on plants until the plants are completely dead will reduce tuber infections.

F. Protect plants with fungicides

Begin the protectant program when plants are about 6 inches tall or after the first cultivation. If conditions are ideal for late blight early in the season you should reduce your spray schedule to 5-day intervals to protect the newly developed leaves.

There are several protectant fungicides available. See “Specifics of fungicides to select from” on page 39. It is wise to have more than one available for use during the season. Some pathogens may become resistant to a family of chemicals or you may run into chemical use restrictions such as the amount per acre one can legally apply during the growing season. The first five categories have efficacy for both early and late blight.

• EBDC products (Dithane, Penncozeb, Manzate, Polyram and Maneb)
• Chlorothalonil (Bravo, Equus, Echo)
• Tin- based products (Super Tin, Agri Tin)
• Copper-based products (Kocide, Champ and others)
• Strobilurin (Quadris, Amistar Gem, Headline) - Excellent control of early blight and has activity towards late
blight.
• Fungicides specific for late blight:

o Cymoxanil (Curzate) - The cymoxanil compound provides the kickback activity from the time of application. The kickback is effective for 24 hours post application.
o Propamocarb (Previcur) - Propamocarb has a limited anti-sporulant activity and may be able to hold the infection for several days. However, the infection is not cured.
o Dimethomorph (Acrobat) - Dimethomorph has an anti-sporulant activity.

G. Spray Equipment

Fungicides applied with high-volume boom sprayers are most effective. These should be operated at ground speed of 4 mph or less, using 50-60 gal/A, and with 60-100 psi of pressure. Hollow cone or flat fan nozzles give the best coverage.

H. Management of late blight for fields in close proximity to a late blight outbreak

Curzate will give some “kickback activity” in case spores already reached your field. Curzate no longer has mancozeb mixed with it so use a protectant companion such as mancozeb or chlorothalonil. Use a 5-7 day spray schedule depending on the weather conditions. Alternate the chemicals, for example use Curzate and a companion fungicide for the first week and the following week use one of the companion protectants alone. The label for Curzate allows a maximum of 5 applications. There is also Previcur (former Tattoo without the chlorothalonil) (maximum of 11.5 pt/A) and Acrobat (also without the mancozeb) (maximum of 5 applications). Curzate performs best with very actively growing plants, whereas Previcur will work even on much older plants. Therefore save the Previcur for later in the season.

I. Harvest only after vines are dead

Harvest when the soil is dry. Also, those fields with high incidence of late blight that might have resulted in tuber rot should be harvested last. Under situations without late blight when other soft rots are prevalent, harvest those fields last. This will allow rotting potatoes to decay prior to harvest. Do not harvest rotting potatoes. Rogue out blighted tubers at grading. Take every feasible step to keep rotting potatoes out of your storage.

J. Manage the storage environment to suppress late blight and soft rots

Manage storage conditions to promote complete suberization of harvest wounds then lower the temperature to appropriate long-term storage conditions. Do not attempt to store lots that have greater than 2% blighted tubers. Avoid placing wet tubers into storage. Moisture provides conditions promoting rot. Good air circulation is required to prevent wet pockets. Also, relative humidity should be below 85% to prevent condensation on the tubers.

K. Seed Treatments vs. In-furrow treatments:

There are several seed treatments that will reduce Rhizoctonia (black scurf). The best control of Rhizoctonia would be both a seed treatment and an in-furrow application of Quadris. Seed treatments with the highest activity towards Rhizoctonia are Maxim MZ or MonCoat MZ. Tops MZ and Evolve are the seed treatments of choice if seed borne late blight is a concern. There are no effective seed treatments of silver scurf. If Quadris is used in-furrow, it does not mix well with Ridomil Gold for the in-furrow application. The end result, if you are not careful is “cottage cheese.” If the two chemicals are to be mixed, make a slurry of each chemical and gradually mix the two.

Ridomil Gold used in–furrow may not be the best use of the chemical. Keep in mind it is systemic and is expected to be taken up by the plant and the tubers. However, by applying Ridomil early in the season at planting especially for indeterminant or late maturing varieties, the chemical will only be found in the first formed tubers and not in those formed later in the season. Therefore you will not have full protection of all the tubers toward pink rot or Pythium leak.

L. Resistance Management:

There are seed treatments and in-furrow fungicides that are the same chemistry, such as Moncoat MZ (seed treatment) and Moncut
(in-furrow). Make sure that you do not follow Moncoat MZ with a Moncut in-furrow treatment.

There are several foliar fungicides that have the same chemistry, such as the strobilurins. Fungicides such as Azoxystrobin, Pyraclostrobin, and Trifloxystrobin belong to the Quinone Outside Inhibitor (QoI) target site group. This group has a high risk for the development of resistance, this means that pathogen populations could lose sensitivity to these fungicides. Therefore follow the label carefully. These fungicides should be alternated with a standard protectant, such as chlorothalonil, mancozeb, or a fungicide from another chemical class. Fungicides from within this group (group 11) should not be used with each other. (Do not apply 6 applications of one strobilurin and then shift to a second one.) Here are some guidelines for strobilurin use:

1) Do not use a strobilurin alone without having a companion protectant.
2) Do not make sequential applications of a strobilurin fungicide.
3) If applying a strobilurin, no more than 1/3 of the total season’s fungicide applications may be a strobilurin.
4) There are also in-furrow and foliar fungicides of the same chemistry especially in group 11, the strobilurins. If a strobilurin is used in-furrow, do not apply a strobilurin fungicide as the first foliar spray.

The following fungicides are medium risk for resistance:

• Acrobat (dimethomorph) (Group 15)
• Curzate (cymoxanil) (Group 27)
• Previcur Flex (propamocarb hydrochloride) (Group 28)

Tank mix these with an appropriate protectant fungicide. Rotate these chemicals such that after 3 applications are made of one chemical, switch to another.

For multiple-site fungicides that are low risk, such as mancozeb (group M3), chlorothalonil (group M5), and SuperTin (group 30), rotate among classes of fungicides, especially if early blight has not been controlled in the past.

M. Integrated Pest Management (IPM):

Another facet to disease management is to select varieties that have some level of resistance. If a variety is listed as resistant that does not necessarily mean the disease will not occur. Therefore all other appropriate management procedures should be followed. However, fungicide usage may be reduced to maintain adequate management of the disease. See the table below for varieties and their susceptibility.

The best management of any disease will incorporate scouting, weather monitoring, performing appropriate cultural practices combined with a fungicide program. A fungicide program using protectants for the majority of the applications with one of the late blight specific chemicals for critical periods should do a good job.

Early blight is the second most important foliar disease on potatoes. If fungicides are applied to control late blight, early blight may also be controlled. The best fungicides for early blight control are Quadris, mancozebs, manebs, chlorothalonil and metiram.

Most other potato diseases are managed by a combination of fungicide seed treatments, resistant varieties, manipulating the environment and performing appropriate cultural practices. Manipulating the environment includes proper irrigation management.