CIS 993 Management of vegetable diseases in home gardens

W. M. Colt, S. K. Mohan, and S. L. Hafez

Plant disease management starts the garden each year will increase ply centers. Occasionally, the long before you vegetable populations of disease-causing home may want to sani- or seedlings. It requires an organisms specific to those . tize small quantities of soil. If you understanding of the nature of the For example, do not follow do, then use one of the following disease and implementation of a beans with peas or potatoes with methods. prevention and control program. eggplant because the fungi in the Dry heat To reduce losses from plant soil that causes damping-off, diseases in your garden, take the rot, or wilt will increase in popula- Oven sterilization — Place soil following precautions and steps. tion and may severely damage the not more than 4 inches deep in following crop. Generally, disease- nonplastic containers such as Dispose of plant causing organisms that attack flats, clay pots, and glass or metal baking pans. Level the soil, but do residue at the end of monocot plants such as sweet corn do not attack broad-leaved plants not pack it. Tightly cover each 1each season. such as tomato and vice versa. container with aluminum foil. In- You may incorporate plant resi- sert a meat or candy thermometer due into the soil early in the fall so Use sterilized through the foil into the center of the soil. it decomposes, remove it from the potting mixes Set oven temperature between garden and put it in a compost pile, or treat small or destroy it. 3 180° and 200°F. Place containers Before returning plant residue to quantities of soil in oven and heat for 30 minutes your garden, be sure it has decom- for transplanting. after the soil temperature reaches ° posed completely. Many plant 180 F. Do not let the temperature Pathogen-free potting ° disease-causing organisms survive go over 200 F because this may soil is desirable for generate products toxic to in dead, nondecomposed plant houseplants and material, and some can survive plants. Let the soil for starting trans- cool and remove the even if the plant material has de- plants. You may composed completely. Therefore, containers from buy sterilized pot- the oven. do not include diseased plant ting mixes at material in the compost pile. garden sup- Adopt a long-range crop rotation 2 program. Growing the same or closely related plants in the same area of

Cooperative Extension System ❏ Agricultural Experiment Station Leave the aluminum foil in place Precautions items that will come in contact until ready for use. Avoid toxicity from heated with the treated soil with a 1:10 dilution of household bleach. Moist heat soil — In heavier soils and soils containing high levels of organic Pressure cooker sterilization — matter such as manure, compost, Treat garden site Pour several cups of water into a leafmold, or peat moss, heat sani- pressure cooker. In shallow con- soil if necessary tation may produce a toxic effect before planting. tainers, place soil no more than 4 that may cause poor seed germina- 4 inches deep. Level the soil, but do tion, plant growth abnormalities, Disease-causing organisms such not compact it. Cover each con- or plant death. as fungi, bacteria, and nematodes, tainer with aluminum foil. An accumulation of ammonium, as well as insects, may build up in Stack the containers on the rack soluble organic compounds, min- garden soil and prevent satisfac- in the cooker. Separate the con- erals, or salts causes the toxicity. tory growth of many different tainers to allow steam circulation. Soil toxicity is most likely to occur plants. In this case, some form of Close the lid, but leave the steam if you heat the soil too long or at physical or chemical soil treatment valve open somewhat until all the too high a temperature. may be necessary before planting. air is forced out and steam begins A heavy irrigation will leach out to escape. Then close the steam Soil solarization many of the toxic substances from valve and heat at 10 pounds pres- Solar heating the soil is one way such soil. Also, storing the soil 2 to sure for 15 minutes. Turn off heat, to control soilborne pathogens, in- 3 weeks without the aluminum allow the containers to cool, and sects, nematodes, and weeds. Just cover will reduce the toxicity. remove them. Leave the aluminum mulch thoroughly cultivated and foil on until you are ready to use Prevent recontamination of moistened soil with a transparent the soil. treated soil — Heat treatment of polyethylene sheet during the hot soil reduces or destroys not only Steam sterilization without season. The soil temperature in- the harmful pests, but also the pressure — Pour about an inch of creases primarily because of the beneficial organisms that are natu- water into the sterilizing container. elimination of evaporation and par- ral enemies or predators of the Follow the soil preparation proce- tially because the polyethylene disease-causing organisms. If heat- dures above. film creates a effect. treated soil is recontaminated with The soil solarization process Place filled soil containers on a plant disease-causing organisms, rack that will keep them out of the generates moist heat causing com- they will grow and reproduce rap- plex changes in soil that are effec- water. Close the lid and idly, reaching high population bring the water to a boil. tive in controlling many plant levels in the absence of natural pathogens and pests while stimu- Open the lid just enough to enemies. prevent the pressure from lating activity of soil microorgan- Do not place plants from con- building up. When the isms beneficial to plant growth. taminated soil or contaminated soil steam begins to escape, Soil solarization, however, is not itself in treated soil. Use only continue boiling for 30 effective against all disease- disease-free planting material in minutes. Then turn causing organisms and is practi- treated soil. Sanitize all off the heat, close cable only in warmer climates. cultivating tools, the lid, and remove pots, flats, and Soil fumigation the soil containers any other when cool. When certain chemicals are introduced into the soil, they are converted into toxic gases killing weeds, insects, and disease- causing organisms. Currently, there are no products available for use by the home gardener. Only someone with a commercial pesti- cide applicator’s license may apply registered fumigants. Prepare seedbed substantially reduce the yield and Irrigation quality of tubers produced from and select disease- Vegetables usually are shallow- such planting stock. rooted plants. Soil that is too dry 5 resistant varieties. To be labeled “certified,” potato may limit plant growth seriously. seed tubers must meet strict stan- Prepare the soil Any irrigation method is satis- dards of freedom from these patho- factory if you apply water prop- Good seedbed preparation is gens. Also, “noncertified” potato erly. Avoid rapid application rates important and cannot be over em- seed may introduce plant disease- that may cause water run-off. Light phasized. Seeds placed in moist, causing fungi, bacteria, viruses, or and frequent water applications are finely granulated, firm soil germi- nematodes into your garden site. best when starting seedlings. As nate readily. Small seeds placed in the plants grow, apply just enough cloddy soil often germinate poorly Treat the seed moisture at each irrigation to wet or die soon after be- Most soils and some seeds con- the root zone of the plant. cause the soil dries before the tain plant disease-causing - Plants with shallow root systems become established. isms capable of causing seeds to become stressed easily during hot decay (seed rot) or young seed- weather. Shallow roots develop Plant resistant varieties lings to die (damping off). These from excessively wet conditions Obtain seed or seedlings of diseases are more severe if garden and from frequent light irrigations disease-resistant varieties. Many crops are planted early in the sea- that do not completely wet the vegetable and flower varieties are son when the soil is cold and wet. entire rooting depth. resistant to one or more specific Planting treated seed often will diseases such as wilts, rusts, mil- ensure good stands and avoid the Generally, irrigate light sandy dews, leaf spots, viruses, or nema- time and expense of replanting. soils one or more times per week and heavy soils every 10 days to 2 todes. Check the seed packet or Most of the seed on the market weeks depending upon the wea- with the nursery for the variety’s is treated and stamped “treated.” If ther. Plants will transpire two to disease reaction. you buy untreated seed, you may three times more moisture on hot, treat it with recommended seed Plant disease-free seed windy days than on cool, cloudy, treatment chemicals. Place the or transplants windless days. seed in a jar, spread the required Obtain disease-free seed or amount of chemical over the seed, Keep the soil moisture supply as transplants from nurseries or and shake or rotate the jar to coat constant as possible throughout the seedsmen with a reputation for each seed with the chemical. When growing season. Roots do not seek producing high quality transplants using any seed treatment chemical, out water. Instead, roots will grow or seeds. Many plant disease- read and follow the label directions only when water is readily avail- causing organisms are capable of carefully. able. Plants wilting from lack of living on or in the seed. water already have been under Unless the seed is disease-free, water stress for 2 or more days. it is not wise to save seed from One way to tell if it is time to your own garden. Also, do not Follow good irrigate is to dig into the root zone. save seed from F1 hybrids such as cultural practices. If the soil holds firmly together tomatoes. If you save and plant 6 when gently pressed in your hand, seed from F1 hybrids, the resulting Cultural practices influence the it is moist enough. If the soil plants will be “interesting” to ob- incidence and severity of plant crumbles easily, it is too dry. serve and may look a little odd, but diseases. Any practice that causes Remember, dig to the root zone. they will not yield vegetables of stress to plants predisposes those This will be the same quality as the parent plants to diseases. The higher the different plants. number and frequency of stress depths Also, be sure to plant only certi- factors, the more severe the dis- fied potato seed tubers. Potatoes eases. from the grocery store are edible, In the home garden the most but they often are infected with common stress factors are irriga- disease-causing organisms that tion, fertilization, and cultivation. for different plants. This is why it infection to occur. Pesticides are naturally occur- is wise to grow all shallow-rooted Increase row spacing, orient the ring or synthetic products used to plants in one part of the garden and rows parallel to prevailing winds, control such pests as weeds, in- those that develop deep root sys- and irrigate or sprinkle only when sects, nematodes, and disease- tems in another part. A mulch of temperatures are rising (usually in causing organisms. Most pesticides compost or grass clippings helps the morning) to minimize the time for home garden use are called retain moisture and hold down soil foliage remains wet. Avoid work- protectants and prevent infection temperature. A black plastic mulch ing among the plants when the of the plant. Apply them promptly also will help retain moisture, but leaves are wet because this facili- and before disease buildup. For will increase the soil temperature. tates disease spread from plant to good control, you should reapply plant. Also, wider spacing of pesticides every 7 to 14 days. Fertilization plants in the rows may help reduce Dusts, liquids, or wettable pow- Different plants require varying or prevent disease spread. ders are available, but be careful in amounts of nutrition for maximum Support plants to prevent fruits selecting the proper formulation. production and quality. Nutrient from coming in contact with the When you apply chemicals, be excess as well as deficiency will soil. Wire cages, plastic, or organic sure to adequately cover all plant put plants in stress. To determine mulches will reduce many fruit parts. the type and amount of fertilizer rots. Avoid injury to fruits. Cuts, Use only pesticides registered needed, send a soil sample to any bruises, cracks, and insect damage for the crop and the disease that soil testing laboratory. are often sites for invasion by they are intended to be used. Rec- Cultivation disease-causing organisms. ommendations for control of dis- eases in the home vegetable garden Cultivation should be shallow Store only those vegetables that are in the publication, Pacific and frequent. Keep the soil from are free from injury or disease. Northwest Plant Disease Control becoming packed and soggy. Cul- Place them in a dry area or under Handbook. Revised annually, the tivation after rains or sprinkling refrigeration to prevent the devel- handbook contains available la- controls weeds and keeps the soil opment of rots or molds caused by belled products, proper rates and surface from crusting. fungi or bacteria. intervals between applications, and Your goal should be a weedless Use pesticides to the necessary waiting period after garden. Weeds take away valuable application and before harvest can plant nutrients, reduce the moisture control some begin. To buy a copy, contact the supply, are unsightly, and may diseases. Ag Communications Center at harbor many plant disease-causing 7 You can manage many plant Oregon State University by calling organisms and insect pests. Also, (503) 737-2573. weed control is important because diseases with appropriate cultural Always read the pesticide label weeds slow air movement around practices and without pesticides. and follow the directions. Handle the desirable plants and favor dis- Under some conditions, however, pesticides safely, and store and ease development. you may need to apply pesticides to control certain diseases. dispose of containers properly. Other cultural techniques Keep plant foliage as dry as possible for as long as possible. Bacteria and fungi (except for The authors powdery mildews) require W. Michael Colt is an Extension horticulturalist, S. free moisture on the foli- Krishna Mohan is an Extension plant pathologist, and Saad age for 3 to 12 hours for L. Hafez is Extension nemotologist all at the University of Idaho Parma Research and Extension Center.

Issued in furtherance of cooperative extension work in and home economics, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, LeRoy D. Luft, Director of Cooperative Extension System, University of Idaho, Moscow, Idaho 83844. We offer educational programs, activities, and materials without regard to race, color, religion, national origin, gender, age, or disability, in accordance with state and federal laws. 2,000 6-93 (replaces CIS 283 and 700) 50 cents