The Pennsylvania State University
Cooperative Extension

Frequently Asked Questions About Late Blight

A lot of questions about late blight have been raised by both home owners and growers. Meg McGrath has addressed some of these frequently asked questions below. Although these responses are geared towards home owners there is still information that is relevant to growers. Additional frequently asked questions will be addressed in future updates.

From Margaret Tuttle McGrath, Department of Plant Pathology and Plant-Microbe Biology, Cornell University, Long Island Horticultural Research and Extension Center, Riverhead, NY with additional information contributed by Sara May, Coordinator, Plant Disease Clinic, Department of Plant Pathology, The Pennsylvania State University, University Park, PA.

For more information about late blight and other vegetable diseases, please visit Penn State Cooperative Extension's Vegetable Pathology home page.

1. Are all tomatoes and potatoes in the northeastern US doomed to get late blight this season because of the wet weather?

No. In addition to requiring that there be favorable conditions (cloudy, rainy, and not too hot) and a susceptible plant (tomato, potato, and some related weeds), late blight cannot develop unless the pathogen that causes this disease is also present. This disease normally occurs sporadically in the northeast, and rarely in many parts, because the pathogen is usually not present. For late blight to develop in a particular garden or field the pathogen has to be there, which it accomplishes by being brought in on infested potato seed tuber pieces or infected tomato transplants or blown in as spores from affected plants in another location. Some plants may ‘escape' late blight if the pathogen does not get on them. However, the chances of this happening this year are very low based on the fact occurrence of late blight is very widespread very early in the summer growing season.

2. Can plants be saved in a garden once late blight starts to develop?

This depends on amount of symptoms seen, type of symptoms, how early in disease development symptoms were found, environmental conditions, proximity to other gardens or farms where late blight is developing, and management steps being taken. It is more likely possible to save plants in a garden if when the first symptoms are found 1) there are very few, 2) they are on the leaves and not stems, 3) the garden has been being inspected very thoroughly on a frequent basis (preferably daily) and thus the symptoms are discovered shortly after they formed, 4) conditions are expected to be hot with no rain or lengthy dew for a prolonged period, 5) there are no nearby places with late blight that could be a source for more spores, AND 6) further development of late blight will be slowed by regularly removing affected tissue (daily cut off and bag, preferably during the day when plants are dry and there will be sunshine for several hours afterwards) and applying fungicides (minimum of weekly). Additionally, success is more likely if fungicides were applied before symptoms were seen (thus there will be fewer initial symptoms) and spray coverage is maximized by using a pressurized pesticide sprayer to plants that are trellised. Removing extra branches will also help. Realize that even with an ideal situation (all above conditions met) there is no guarantee that success will be achieved. Late blight is a very destructive and difficult to manage disease. Impact can be great considering that tomato fruit and potato tubers that become infected can quickly rot. Plants can be killed quickly when late blight is not managed. A spot (lesion) can form within 4 days of when a spore lands on a plant (even faster, less than 3 days, with one strain of this pathogen) and a day later be producing spores that can be wind dispersed to healthy plant tissue resulting in more spots within a few days. Lesions that develop on stems are especially destructive. Late blight needs to be aggressively managed not only to try to save plants in the garden but also to avoid having the affected plants serve as a source of inoculum (wind-dispersed spores) for other gardens and farms. Promptly remove affected plant tissue on a regular basis. Realize that when symptoms are first seen, all points of infection likely are not yet visible. There is a few day ‘latent' period between infection and when symptoms are visible. No fungicide can cure tissue that is already affected, and this tissue will produce more inoculum. Also rogue out any volunteer tomato plants growing from seed of previous year's tomatoes and susceptible weeds like bittersweet nightshade. Given the amount of effort to try to save a garden once late blight starts to develop, especially when it is early in the season, and the chance the crop will be destroyed despite the effort, especially if fungicides are not applied frequently, the best option when late blight occurs might be to replace the plants with something like spinach or lettuce that grows quickly.

3. Can plants be saved in a farm planting once late blight starts to develop?

Yes. Potato growers usually are able to effectively control late blight. It is easier to manage late blight on a farm than in a garden because of the fungicides that can be used. Farmers can use fungicides able to move within the leaf that the spray lands on; some fungicides can move into stems and new growth. Farmers also have sprayers that can achieve better coverage of plant tissue than hand sprayers. As with a garden, success is affected by whether or not fungicides were applied before symptoms were seen and how severely the crop is initially affected if fungicides were not applied. Typically farmers begin applying a broad-spectrum, contact fungicide when conditions are favorable for late blight, inspect their crops regularly, and when symptoms are found start applying fungicides with specific activity for late blight. They also manage the usual initial source of the pathogen: affected tubers from the previous year or used as seed. See also answer to previous question.

4. Do I really need to apply fungicides preventively to control late blight?

If fungicides are not applied preventively, there is a risk that when this disease begins to develop, there will be too many symptoms to achieve control. Fungicides cannot ‘cure' a spot that has already developed (disease control in plants is very different from humans). This tissue will soon die, but before it does the pathogen will produce hundreds of spores. The more spores, the greater the odds some will be dispersed to plant tissue that has not received fungicide. It is difficult to achieve complete coverage of plant tissue with fungicide, especially when a contact* fungicide is used, even with the best farm sprayer. The underside of leaves is an especially difficult area to reach (the pathogen can infect through either surface), which is why farmers who can use fungicides able to move through leaves are better able to control late blight. Only contact fungicides are available to gardeners. Note that there are precautions that need to be taken when applying fungicides. Read the label to determine what protective equipment is required (e.g. water-proof or chemical-proof gloves, shoes plus socks, long pants, long-sleeved shirt, goggles, respirator).

5. Do I really need to apply fungicides frequently to control late blight?  

Yes. Fungicides applied on a plant, even those that get inside of the plant, disappear over time due to being broken down biologically or by sunlight and/or being washed off by rain or irrigation. After about 7 days the concentration (dose) of many fungicides can be too low to be adequately effective.

6. What other diseases could be confused for late blight?

There are several diseases that can cause dark spots on leaves and stems of tomato and potato plants. Most of these are smaller than those due to late blight. Botrytis gray mold is the most similar.

7. Could the late blight pathogen survive in soil between seasons?

Unlikely except in affected potato tubers. This is an obligate pathogen that is thought to only be able to survive in living plant tissue in the northeastern US. It can produce a specialized structure that would enable it to survive without living plant tissue, but this requires that the pathogen reproduce sexually which it is not thought to be able to do in the northeastern US. When late blight has previously developed in this region and the pathogen population has been examined, only one “mating type” has been found. This is the term used for the pathogen's equivalent of male/female. Thus the pathogen has only been able to reproduce asexually. The characteristic white growth that develops on late blight affected tissue contains many asexually-produced spores. Both mating types have been found in Florida.

8. Could the late blight pathogen survive between seasons on perennial weeds that it is able to infect (e.g. bittersweet nightshade and hairy nightshade)?

No. This is an obligate pathogen that needs living plant tissue to survive. It only infects foliar tissue of weeds. Since the pathogen cannot infect roots, it cannot survive on weeds in areas where foliage is killed by cold temperatures. Potato tuber is the only plant tissue it is able to survive in.

9. Are affected tomatoes safe to eat?

Yes. This pathogen cannot infect people and it does not produce a toxin that can make people sick, as a few plant pathogens can do. However, considering how quickly affected tomatoes will rot, it is unlikely that they will be marketed. For more information about eating or canning affected tomatoes, please see http://foodsafety.psu.edu/LateBlight.htm.

10. Are there resistant varieties?

Not yet, but there are some in the final stages of development expected to be available perhaps as soon as 2010.

11. How should diseased plants be disposed of?

Diseased plants should be placed in trash bags and disposed of with the household's regular refuse or alternately, be buried. Diseased plants should not be used as composting material.

*Contact fungicides remain on the surface of the plant tissue where they are deposited whereas translaminar fungicides can move in and through a leaf. A very few fungicides are systemic and can move in the plant to stems and new growth. In contrast, most human drugs are put inside the body and are able to move to where they are needed and they have a curative effect. Plant medicine is very different from human medicine.