Managing diseases and pests the home vegetable garden.
1. Plant Selection and Health:
Many problems can be avoided if we start with appropriate seeds and transplants and keep them healthy with proper nutrition and soil moisture levels. For certain pests and diseases, cultivars are available that are resistant or tolerant. Gardeners are often advised to select varieties with disease resistance but seed catalogues either do not list this information in the variety descriptions or use vague wording such as disease resistant or disease tolerant. A perusal of two Canadian seed catalogues revealed that one gave quite complete listings of disease resistance for many varieties whereas another catalogue usually omitted the information.
2. General Management:
Practices that can make your garden less susceptible to plant diseases Include:
* Planting the garden in an open sunny area.
* Avoiding close spacing, which reduces air movement and keeps leaves wet for extended periods.
* Avoiding high rates of nitrogen fertilizer that promote a dense foliage growth and conditions favourable for disease spore germination. Use a balanced slow release fertilizer.
* Watering in the morning allows the plant to dry off during the day. Late evening watering can promote leaf diseases such as early blight, late blight and powdery mildew.
* Mulching is generally useful for conserving soil moisture and protecting perennial roots from severe winter temperatures, but in some areas it can exacerbate a problem with slugs. Maintaining an area of undisturbed litter on the ground surface around trees and shrubs may encourage the increase of ground beetles, predators of slugs (The 200 Prairie Garden, p.112).
* The moths of several species of cutworms lay their eggs in loose, dry, surface soil in gardens during August and early September. Cultivation in September / October buries eggs and reduces their survival, thus reducing the numbers of cutworm larvae that hatch in the spring.
Even healthy plants can be attacked so continued vigilance is needed to detect and identify problems at an early stage. A healthy yard is home to many species of insects, most of them benign or beneficial. It is important when confronted by a "diseased" leaf or plant, to be able to distinguish if the symptoms are caused by insects, an infectious bacteria or fungus, or mineral deficiencies related to poor soil conditions. The excellent book Diseases and Pests of Vegetable Crops in Canada (R.J. Howard, J.A. Garland & W.L. Seaman (Eds.), 1994) has good colour photos and full descriptions of diseases, pest insects, and some beneficial insects. Diseases and pest insects can usually be identified by comparison with the descriptions and photos in such a publication. However, it may be necessary to submit samples or specimens to a diagnostic facility. Information about these facilities may be obtained from your provincial department of agriculture, university departments or garden centres. They should be able to give you an address and the amount of a fee for identification, if applicable.
Sometimes, we must make a choice between values: the beautiful black swallowtail butterfly adorns our gardens as it flits about feeding on nectar, but its larvae will defoliate dill, parsley and occasionally carrot and parsnip plants (The 200 Prairie Garden, p.87). If you like the butterflies, don't kill the larvae.
Remove infested plant material. The spread of disease-causing organisms and insects can be retarded by the prompt removal and destruction of infested plant parts and plants. Leaves that are infested with potato beetle eggs or those heavily infested by aphids can be removed. Finding and killing a cutworm larva as soon as it eats the first plant is also effective in minimizing loss. Composting diseased material is not recommended because composting may not destroy all of the disease organisms. Good sanitation reduces the chances of infestation and spread, and is useful regardless of the size of your garden.
5. Crop Rotation:
Crop rotation is a viable technique for minimizing attacks by many insects and diseases, particularly those that overwinter in the soil or affected plant parts. These need to move to a new host for the next summer. Distance from the infected area and time, 2 to 5 years, are needed to avoid re-infestation.
Rotation is of limited use to home gardeners because their gardens are too small to allow either a sufficient distance to prevent migration into the new crop, or enough time before the same crop is replanted on the infected land. In very large gardens, you can rotate among family groups of vegetables: crucifers (cabbage, turnips, broccoli and other cole crops); legumes (beans, peas); umbellifers (carrots, parsnips, dill, parsley); and lilies (onions, garlic). One crop that can be effectively used in a garden rotation is corn. Corn, a member of the grass family, is not related to any other garden vegetable, and does not share any disease problem with other vegetables. Rotation of these 5 groups allows 4 years before putting a group back on a piece of ground, enough time to kill most soil inhabiting disease organisms.
The diseases most effectively controlled by crop rotation are root pathogens such as fusarium and verticillium wilt. However, long rotations of at least 3 or 4 years are required to reduce the incidence of disease inoculum such as the thick-walled resting spores of fusarium or the microsclerotia of verticillium. As well, mixing of soil such as might occur in rototilling can contaminate previously unaffected areas of the garden. Root pathogens like fusarium tend to infect plants within the same family, e.g., tomatoes and potatoes, cabbage, cauliflower and broccoli, or beans and peas. Verticillium is not as host-specific and can infect potatoes, tomatoes, sunflowers and legumes, thus reducing the effectiveness of crop rotation.
Crop rotation is not an effective control of leaf and stem blights where spread can occur through wind and water splashed spores. This group of diseases can be divided into 2 categories. Those such as early blight of tomatoes and potatoes overwinter locally on infected crop debris. Destroying dead tomato and potato vines in the autumn can reduce early blight. Localized re-introduction of the disease can occur from diseased crop debris overwintering in adjacent gardens and cropland. While crop rotation is not effective, other control measures can be taken. Protective fungicide applications must be repeated at 7 to 10 day intervals beginning in early July and continuing until harvest to be effective.
Diseases such as late blight are re-introduced each year by wind blown spores originating in southern areas. This is also the case with aster yellows in crops like carrots where the disease organism is reintroduced each year by means of infected leafhoppers from the US. The diamond-back moth, which feeds on crucifers, also comes in on southerly winds in the spring, but later generations spread by local flight.
Although late blight of potatoes and tomatoes is usually reintroduced each year, some localized disease carry over can occur through volunteer potato plants. Volunteer potatoes rarely occur in prairie gardens, and only in years where there is an early and lasting snow cover before the ground is frozen. Volunteer plants should be destroyed in the spring before emergence of the newly planted potatoes. Crop rotation is not an effective control. Other control measures include treating potato seed pieces with a fungicide, and repeated fungicide applications as recommended for early blight.
Some diseases such as white mould of beans have a very broad host range and combine a soil-borne inoculum stage (i.e., a sclerotia with a wind borne spore; ascospores from germinating sclerotia). Small garden plot rotations are not an effective means of controlling sclerotinia. The same strain of the disease can infect beans, peas, cucumbers, tomatoes, potatoes and sunflowers making it difficult to rotate. Sclerotia produced within infected stems can overwinter and remain viable for up to 4 years. The sclerotia germinate and produce spore-producing structures following a wet period of at least 10 days where the top 10 cm of soil remains moist. Disease control measures that can be taken against sclerotinia include wide row spacing to allow drying of the soil surface and avoiding frequent low volume watering that perpetuates a wet soil surface. Watering should be less frequent and heavier.
Rotation is not effective against several insects that may overwinter in our gardens. The cabbage root maggot that attacks crucifers and the onion bulb fly and onion maggot that harms onions overwinter as pupae in the soil. The adults that emerge the next year can fly more than 1 km to find new hosts. Therefore crop rotation within a home garden is ineffective. Not growing the affected crop for 3 to 4 years will reduce and possibly eliminate the problem, unless a neighbour shares the problem and continues to grow susceptible vegetables.
Insects that can move long distances are similarly unaffected by rotation of vegetable crops. Crucifer flea beetles do not usually reproduce in home gardens but can severely defoliate horseradish and cole crops in the spring before canola crops emerge, and in August after they are harvested. These beetles can find even a single plant or small row of crucifers and can devastate newly germinated radish seedlings or cabbage and cauliflower transplants. In hot, dry years, the damage can be done in a few hours, so insecticidal treatment is usually more for revenge than protection. Covering newly emerged plants with protective cloth (e.g., "garden blankets") works, but only if the edges are well closed. Cutworm moths also can fly long distances to lay their eggs in your garden. Late cultivation may bury eggs and young larvae and thus reduce cutworm numbers in the following year. Adult potato beetles and sunflower beetles also can move several kilometres and are not affected by home garden practices.
Reduced losses from several insect pests have been achieved in field-scale experiments with several crops. However, unless you are growing only one crop in your home garden, you already have a degree of interplanting far more varied than in any of these experiments. There is no evidence that changing from alternating rows to alternating smaller units will decrease the ability of pests to find a host in your garden.
In spite of all preventative measures taken against pest insects and plant diseases, pesticide applications may be necessary to save the crop. If you think you need to apply a pesticide, be sure of the pest identification, determine the appropriate pesticide, and follow the directions carefully. Although all pesticides recommended for domestic garden use have been tested and found safe to use when applied following the label recommendations, some gardeners may prefer to use organics. Sulfur and fixed copper fungicides are generally acceptable in organic production as an alternative to synthetic fungicides. There are anecdotal reports that garlic naturally contains high levels of sulphur and a few cloves crushed in water can be used to make a homemade spray. Proper timing of fungicide applications is important for successful disease control. Begin at the first sign of the disease and repeat at regular intervals of 7 to 10 days if weather conditions favourable for disease development persist. Most diseases are favoured by prolonged periods of cloudy and humid weather.
Home gardeners on the prairies have fewer insect and disease problems than gardeners in warmer climates. In general, good sanitation and management practices are sufficient. With regard to insects, excellent advice on the identification of beneficial, benign, and pest insects and responses to their presence is given by The Prairie Gardeners Book of Bugs: A Guide to Living with Common Garden Insects by N. Bryan & R. Staal, 2003 (see review in The 2004 Prairie Garden, p.72). The insects in your vegetable garden, like those in your yard, should be looked upon as a source of pleasure, so relax and think before you start killing.
Bill Turnock is a member of The Prairie Garden Committee and a retired research entomologist. Gary Platford is the Co-Chair of The Prairie Garden Committee and a retired plant pathologist.