4 pp., 3 photos, 1 tablePlant diseases can be caused by certain organisms or by environmental factors. This publication discusses non-chemical methods that suppress disease-causing organisms

Kaufman, Harold W.

Philley, George L.

Texas A&amp;M Repository

Plants are considered diseased when they do notgrow and develop normally. Diseases are caused byorganisms (pathogens) such as bacteria, fungi, nema-todes, viruses, viroids, phytoplasmas and parasiticseed plants. There are also environmental disorderscaused by air pollution, poor soil, excessive heat,inadequate or excessive nutrients, and drought. Thispublication discusses only diseases caused bypathogens.Prevention is the basic approach to plant diseasecontrol, whether using chemical or non-chemical(organic) methods. To prevent disease, one mustsuppress the pathogen or eliminate conditions favor-able for disease development. To achieve the bestcontrol, use as many disease-prevention practices aspossible.  With some diseases, certain control prac-tices are more effective than others. For this reason,correct identification of a plant disease is important.Many control practices, however, will help controlmany different disease problems.Suppressing the PathogenRotate crops. Vegetables in the same familygroup (for example, cantaloupes and watermelons)are likely to be susceptible to the same soil-bornediseases. If crops from the same group are plantedone after the other, a disease organism may build upin the soil and cause severe crop loss. To preventthis, rotate vegetables from different family groupsbecause they usually are not susceptible to the samediseases. Rotate crops listed in Table 1 so that selec-tions from one group are not planted in the samelocation more than once every 2 to 3 years. Limitedgarden space may prevent an ideal rotation system.Add organic matter. Organic matter increases thenumber and kind of microorganisms in the soil.Many of these microorganisms compete with diseaseagents for nourishment. In some cases, the bestorganic matter is obtained by turning under a greencover crop such as a small grain (wheat, oats, barley,rye) or a legume. If root knot nematodes are presentin the soil, avoid using a legume as a cover crop.Plant resistant varieties. Agricultural scientistshave made great strides in developing disease-resis-tant varieties.  Resistance, however, is a relative termand does not indicate immunity. For example, atomato plant resistant to Fusarium wilt may developL-201612-99Non-Chemical Control of PlantDiseases in the Home GardenGeorge L. Philley and Harold W. Kaufman**Extension plant pathologists, The Texas A&M University System.Table 1. Crop grouping for rotation to control soil-borne diseases.GROUP A GROUP B GROUP C GROUP D GROUP E GROUP FCantaloupe Brussels sprouts Eggplant Beet Sweet corn BeanCucumber Cabbage Irish potato Carrot CowpeaHoneydew melon Cauliflower Okra Garlic PeasPumpkin Collards Pepper OnionSquash Lettuce Tomato ShallotWatermelon Mustard Sweet potatoRadishRutabagaSpinachSwiss chardTurnipwilt, especially if stressed, but not become as dis-eased as a susceptible variety might. A resistantplant develops disease symptoms more slowly, andmay continue to produce well.Practice good sanitation. Sanitation includesdestroying crop residue so that pathogens can’t sur-vive in the soil and cause a problem the next season.Examples are raking and burning diseased leavesand disposing of infected fruit. If any plants areinfected with root knot nematodes, dig them up andremove as much of the root system as possible fromthe garden site. Root knot nematodes are harder tokill when protected by root tissue.Do not pile diseased leaves, fruits and other plantparts near the garden.  Fungi often produce thou-sands of spores in the reproductive process andthese spores can be wind-blown great distances.Shred diseased plant tissue and till it deeply into thesoil.  Do not put diseased plant parts in a compostpile, especially those with nematode infection.Although many disease organisms are destroyed byheat during composting, some will not be.Contaminated compost may spread disease. Makecompost with grass clippings or shrub and treeleaves, as the disease organisms on turf and orna-mental leaves are not likely to affect garden plants.Treat with heat. Heating small amounts of soil tokill disease organisms is practical for potting plantsand growing seedling transplants.  The soil shouldbe moist but not saturated.  Heat the soil at 180degrees F. for approximately 3 hours in a standardoven. Place an average-sized Irish potato in the mid-dle of the soil to act as an indicator.  When it iscooked, the soil should be sterile. Microwave ovenscan be used for sterilizing small quantities.  Usinghigh power, heat the soil for 3 minutes.  Do not over-heat with either system.  Excessive heating makescertain elements more available and often at toxiclevels.A new heating approach that has shown goodresults in Texas is solarization (soil pasteurization).It consists of covering well-tilled, moist soil withclear plastic, sealing the edges and leaving it on forat least 4 weeks. Do not use black plastic becausethe soil will not heat as well. This method is mosteffective for killing fungi, especially those that causeroot rots. Certain weeds and nematodes are con-trolled, but not consistently. Solarizing whenresidues from cruciferous plants such as cabbage arein the soil improves effectiveness; as cabbage plantsdegrade they produce volatile compounds that aretoxic to some pathogens. Besides killing pathogens,solarization also stimulates plant growth, possiblyby creating a good environment for beneficialmicroorganisms; however, this phenomenon is notfully understood. Solarization works best during thelong, hot days of July and August.  In West Texas,where sunny days are common, solarizing in lateMarch and April can be effective.  Leave land fallow. Leaving land idle and cleanthrough the growing season reduces disease agentsin the soil. Fallowing is especially helpful if done inthe summer months when soil temperatures arehigh.  Frequent tilling or spading keeps the soil dryand free of plant growth and exposes soilborne dis-ease organisms, such as nematodes, to killing heatand excessive drying.  Other benefits of fallowing areweed and insect control. This practice is most effec-tive when rainfall is low and temperatures are high.Like solarization, fallowing is difficult for growerswith small gardens that are intensively cropped.Consider fallowing or solarizing only a portion of thegarden at a time.  Control weeds. Weeds harbor insects and arehosts for many viral diseases. For most viruses tosurvive they must remain in a living organism, eithera host plant or insect, which transmits the virusfrom one plant to another. Destroying weeds in andaround the garden may eliminate potential hostplants. This should be done before planting trans-plants.Avoiding DiseaseChange the planting date. Some diseases are con-trolled by changing the planting date. Spring-plantedsquash usually escapes mosaic virus, whereas fall-planted squash does not. Plant warm-season vegeta-bles, such as peas and okra, after the soil warms suf-ficiently to avoid seedling disease.Leaving land idle and clean through the grow -ing season reduces disease agents in the soil.Fallowing also controls weeds and insects.Obtain high quality planting stock. Check trans-plants such as cabbage, tomatoes, peppers and oth-ers for root knot nematode galls. Certified Irish seedpotatoes are less likely to harbor a disease such asblack leg. Some disease agents are seed transmitted;therefore, plant only the best seed.Use good cultural practices.l Select the best planting site—one with deep,well-drained soil.l Planting on raised beds improves drainage.Raised beds also warm up faster than level soil,so seedlings emerge faster and are more likelyto escape seedling diseases.l Space plants properly so they aren’t too crowd-ed. Crowding reduces air circulation, whichencourages leaf spot diseases such as septorialeaf spot on tomatoes. l Overplanting, where possible, ensures enoughproduction if some plants or fruits are lost todisease.l Fertilize properly. Plants receiving all the nutri-ents they need may resist some disease-causingorganisms. Apply fertilizer based on a soil test.l Do not overwater. Excessively wet soil is morelikely to support soil-borne fungal diseasessuch as root rots and wilts.l Avoid overhead irrigation. Leaf spot diseasesdevelop rapidly when leaves are moist.l Do not prune roots. Cutting roots while cultivat-ing reduces plant vigor and leaves openingswhere root rot and wilt fungi can enter.Control root knot nematode. Root knot nematodeis a major problem in many home gardens state-wide. Some gardens become infested from contami-nated transplants or topsoil. Taking steps to preventthis problem is just as important as implementingcontrols later. When a garden is first planted, if thesoil is infested nematode levels generally are low.After several years of gardening and growing suscep-tible plants, nematode populations increase to thepoint that damage becomes noticeable. Control prac-tices include solarization, summer fallowing, rota-tion, adding organic matter, planting trap crops,removing diseased plants and using tolerant or resis-tant varieties. All these control measures aredesigned to reduce the soil’s nematode population.Benefits of solarization and summer fallowinghave been discussed. Removing host plants andkeeping the soil dry reduce nematode populationsbecause nematodes require a moist environment forsurvival. Although these practices reduce nematodenumbers, the nematodes are not entirely eliminated.Growing one susceptible crop can bring populationsback to damaging levels. Repeated use of these prac-tices is necessary for success.Rotating susceptible crops with non-hosts or poorhosts of the root knot nematode is another means ofreducing the population. Sweet corn is a poor hostand is good to use in a rotation, especially in an areawhere root knot has done severe damage. Onions,garlic, asparagus and shallots also are poor hosts.Cool-season crops such as cabbage, Irish potatoes,greens (turnips), radishes and broccoli are less likelyto suffer yield loss from root knot nematodes. Eventhough these are susceptible plants, they grow bestin cooler temperatures that are not favorable forroot knot nematode development.Just having lots of organic matter in the soil doesnot ensure root knot nematode control. However, themore organic matter there is the better the chancethat antagonistic organisms will develop. Someorganic matter works better than others.  Turningunder a green manure crop such as small grains sev-eral weeks before planting is the best. The crop mayneed additional nitrogen because decay organismsuse available nitrogen as they break down the greenmanure crop.Sometimes people resort to home remedies tocontrol nematodes. These include planting marigoldsor mixing sugar or lye into the soil. Of these three,only marigolds are effective in controlling nematodepopulations, and their effectiveness has limitations.Some people think marigolds secrete a toxic sub-stance that kills nematodes, and that by planting afew marigolds around annual plants in infested soilthey can prevent infection. This is not true.Marigolds may produce some antagonistic chemi-cals, but the primary reason for control is that theyPlanting on raised beds improves drainageand speeds up plant emergence, thus reduc -ing chances of seedling disease.act as a trap crop.  Nematodes enter their roots butare unable to complete their life cycle. Trappednematodes die without reproducing. French marigolds, Tagetes patula, are more effec-tive in controlling root knot nematodes than theAfrican marigold, Tagetes erecta, which also isreferred to as the American, Big or Aztec marigold.To be effective, marigolds must be planted as a solidcrop and grown for 90 to 120 days to reduce thenematode population sufficiently to grow annualplants without treatment. Plant marigolds in rows 7inches apart with 7 inches between each plant sothat roots penetrate the entire soil mass to trap asmany nematodes as possible. If marigolds are plant-ed close together, they form a dense canopy whichretards weed and grass development. Many weedsand grasses serve as hosts for root knot nematodes.If the weeds are not controlled, marigolds will beless likely to suppress the nematode population.Using marigolds in a manner other than that rec-ommended can invite disaster. For example, plantinga few marigolds here and there among tomatoesencourages spider mites. The spider mite is one ofthe most difficult garden pests to control and canbecome nearly as serious a problem as the nema-todes.Remove all nematode-infected plants from the gar-den as soon as possible after production ceases.Removing the root system eliminates many of thenematodes. To remove as much of the root systemas possible, use a shovel rather than just pulling theplant up by the stem.Root knot-resistant vegetable varieties are notplentiful. Fortunately, progress has been made indeveloping root knot-resistant tomatoes. The bestresistance is found in certain hybrid varieties. Rootknot-resistant varieties are noted in seed catalogs by“N” following the variety name. Often “V” and “F”preceed the “N.”  “VFN” stands for Verticillium wiltresistance, Fusarium wilt resistance and nematoderesistance.  Hybrid tomato seed are more expensivethan open pollinated seed, but the benefits in dis-ease resistance alone are enough to justify buyingthem.Remember, for best results in controlling plantdiseases with non-chemical methods, use as many ofthese preventive practices as possible.Suggested ReferencesAgrios, G.N.  1988.  Plant Pathology.  Academic Press,Inc., New York, N.Y.National Academy of Science.  1968.  Plant-DiseaseDevelopment and Control. Publication 1956.National Academy of Sciences. Washington, D.C. Sherf, A.F. and A.A. Macnab.  1986.  VegetableDiseases and Their Control.  John Wiley & Sons,New York, NY.Walker, J.C.  1969.  Plant Pathology.  McGraw-HillBook Co., New York, N.Y.These s wollen and distorted tomato rootsare heavily infected with root knot nema -todes.10M, Reprint PP

Non-Chemical Control of Plant Diseases in the Home Garden

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Non-Chemical Control of Plant Diseases in the Home Garden

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Non-Chemical Control of Plant Diseases in the Home Garden

Abstract

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Texas A&amp;M Repository

Texas A&M Repository