Disease Prevention in Home Vegetable Gardens

Reviewed by David H. Trinklein
Horticulture State Specialist
Division of Plant Sciences & Technology 

Vegetable gardening is the number one hobby in the United States. Additionally, it is an ideal way to save on the family food budget while producing safe, nutritious food (Figure 1). A $70 investment in home food gardening can yield, on average, a $600 return on the investment.

Keeping a garden healthy and attractive requires attention not only to its size and location but also to the soil, water availability, sunlight and air circulation in the garden. These environmental conditions can determine susceptibility to plant diseases. Diseased plants are unsightly and also detract from the enjoyment and fruits of the hobby.

Diseases affect home garden vegetable plants every year. Plant pathogens become established when environmental conditions are favorable. Losses due to disease can be reduced through a combination of proven disease-prevention methods:

• Select adapted, disease-resistant varieties.
• Use transplants that are free from disease.
• Plant closely related vegetables in separate areas of the garden (Table 1).
• Rotate garden areas to prevent planting closely related vegetables in the same area year after year.
• Control weeds that compete with vegetables or harbor plant pathogens.
• Control insects that may carry disease.
• Remove and destroy diseased plant material.
• Remove plant refuse soon after harvest.
• Disinfect garden tools and shears.
• Apply fungicides appropriately and in a timely manner when resistant varieties are not available.
• Maintain a balanced soil fertility program.

In addition to diseases caused by pathogens, many nonparasitic disorders cause serious problems in vegetable production. The following disorders may mimic symptoms caused by pathogens: extremes in temperature, extremes in moisture, extremes in one or more nutrients, and herbicide misapplication or carryover. These disorders will not respond to the use of chemicals aimed at plant pathogens and can make conditions more favorable for disease development.

Table 1. Vegetable families susceptible to similar diseases.

Getting started

Sanitation

Many plant pathogens survive through the winter in old plants and plant debris remaining in the garden. Removal of the plant material will reduce the chance of certain diseases increasing over years. It also reduces the chance that healthy plants will become infested early in the season. Some plant diseases would naturally occur late in the season and not be a problem on older plants. These same diseases can be devastating on young plants if pathogens are present early in the season.

Debris from diseased plants should not be added to a compost pile, because the temperatures reached in the pile often are not sufficient to kill the pathogens. Burying the plant debris outside the garden will reduce the chance of spreading a disease from debris to plants currently in the garden or to plants that will be in the garden the next year. Some pathogens such as the wilt fungi survive in the soil for many years, and prevention is the best way to manage these diseases.

In addition to removing plant material from the garden, it is important to remove, destroy or disinfest support structures such as wooden stakes and poles used in the garden.

Garden tools can be disinfested by washing them with detergent. Washing will remove soil and adhering fungi or bacteria, and the detergent will remove some of the virus from the tools and inactivate any remaining virus.

Clean seeds and transplants help reduce the chance of introducing plant pathogens into the garden. Do not save seed if disease is present in the garden. Whether growing your own transplants or purchasing them, transplants should be carefully inspected for abnormal growth above and below ground. Reject multipacks of transplants if several cells do not have plants or contain dead plants. Inspect transplants for insect damage on the leaf surface or insects on the lower leaf surface. If growing your own transplants, purchase steam-sterilized growth medium. Disinfest flats with bleach or use new plastic containers.

Certain plant pathogens can grow on weeds and spread to garden plants. For example, aster yellows phytoplasma can be spread from dandelions to carrots by the aster leafhopper. Additionally, some weeds attract insects that transmit diseases. This is especially true of viral diseases.

Cultural practices

The garden site should be well drained. Waterlogged soil encourages development of root rotting fungi, whereas good drainage promotes good growth of plant roots and thus the entire plant. If soil drainage is marginal, building a raised bed may solve the drainage problems.

Plants with the proper available nutrients can withstand environmental stresses and plant pathogen attacks better than plants growing in soil with low fertility or where there is a nutrient imbalance.

Plant-parasitic nematodes, especially root-knot, can be a problem in the garden.

Crop rotation is a good way to manage diseases that attack related plants such as tomatoes, potatoes and eggplants (Table 1). Moving the location of the related plants within the garden from season to season lessens the chance that plant diseases will build up. This is especially true of pathogens which survive in the soil. A good rule of thumb is to avoid returning to the same area of the garden for at least three years. This will not prevent diseases with long-lived resting spores, such as Pythium, Fusarium and Rhizoctonia.

Plant at the recommended seeding rate to reduce competition between plants and promote good air circulation and sunlight penetration. Use viable seed with good germination potential. Use seed packaged for the current year. The seed packet should have a date on it.

Physical practices

Plastic sheeting and organic mulch provide a physical barrier between soil and plant surfaces and reduce the amount of disease inoculum splashed onto foliage, stems and fruits during rainy periods.

Staking and trellising

To reduce the incidence of fruit rot in the garden, keep the fruit as far away from the soil as possible. Staking or trellising are especially effective with tomatoes. Sunburn can also be avoided if plants are grown in such a way that the leaves shade the fruit.

Solarization

Soil solarization is a nonchemical way to rid the garden of soil-borne plant pathogens. Solarization uses energy from the sun to heat the soil causing physical, chemical and biological changes in the soil. The process is most effective in mid to late summer, when high air temperatures combine with high radiation from the sun. The elevated temperature and toxic products generated from solarization kill or suppress plant pathogens and weed seed. It is believed that beneficial organisms are harmed less by solarization than by fumigation. Solarization also stimulates release of nutrients from organic matter present in the soil.

The biggest disadvantage to this method is that the area treated must be out of production for most of the growing season. Soil to be solarized should be tilled so that the soil is as uniform as possible (free of clods and plant debris) to prevent pockets of untreated soil. Slight elevation of the treated area will minimize recontamination of treated soil. A raised center of the bed will facilitate rainfall shedding. Water sitting on the plastic reduces the effectiveness of the treatment.

Check soil fertility and, if necessary, add fertilizer before beginning solarization. Dry soil should be moistened to a level that is ideal for planting. Wet soil conducts heat better than dry soil and will allow the heat to move deeper in the soil to remove pathogens present in the root zone.

Use clear plastic (1 to 6 mils) to cover the soil. Thinner plastic allows better solarization. The plastic needs to be stretched tight over the soil surface and be in contact with the soil. It is important to bury the edges of the plastic to prevent easy removal of the plastic before the soil has been adequately treated. Soil temperatures need to be over 100 F for four to six weeks to reduce soil-borne pathogens.

Contaminated plants introduced into the treated soil will undo the effects of solarization. Also mixing of adjacent soil with the treated soil will dilute the benefits of solarization.

Biological practices

Resistant varieties provide one of the best ways to manage plant disease in the garden. Resistance to a disease means that the plant is less likely to show symptoms than susceptible varieties; it does not mean that the plant is immune to that disease. Resistance to one disease does not protect against other diseases. Use of resistant varieties if available is especially recommended when a disease is known to occur in your area. Seed packets, websites and catalogs are good sources of information about disease-resistant varieties. Be sure to check that the variety with disease resistance is adapted to your area before ordering seed. MU Extension publication 6201, Vegetable Planting Calendar lists vegetable varieties recommended for Missouri.

After planting

Sanitation

Make a practice of removing diseased plants or plant parts from the garden without delay. It is often more cost-effective to remove plants than to try to bring them back to health. Removal also helps reduce the chance that disease will spread. Look for leaf spots, wilts, stunting, fruit rots, malformed leaves, and cankers. Bury diseased plant material away from the garden; do not place it in a compost pile.

Many plant pathogens require moisture to survive and infect plants. Avoid working in the garden when foliage is wet, because this can spread plant pathogens.

During the growing season

Good cultural practices

The following practices will help maintain healthy plants during the growing season:

• Maintain adequate levels of plant nutrients without overfertilizing. Excess nitrogen application can promote some root-rotting fungi. Nutrient stress can make plants more susceptible to diseases and insect damage.
• Water when the plants are dry to avoid drought stress. Excess water can lead to plant death from lack of oxygen to the roots or because of pathogen attack.
• Maintain adequate mulch cover to conserve moisture and reduce weed growth. Certain nonparasitic diseases such as blossom end rot can occur when moisture levels to the roots are uneven.
• Harvest produce at peak maturity. Overripe vegetables will attract insects and other pests.
• Remove nonbearing and old plants immediately after harvest to prevent accumulation of plant debris in the garden area.

Chemical control

Sometimes resistant varieties are not available and disease occurs in the garden despite all the cultural practices used. Many leaf diseases can be managed by spraying or dusting plants with an effective fungicide. Most fungicides are protectants. They work on the plant surface and protect against infection. They do not eliminate established infections. If disease is not detected early, the plant may die and disease may spread despite fungicide treatment. Some fungicides are systemic and will move in the plant. Some of these have curative properties and will kill infections already established in the plant, but they will not remove the spots already present on the leaves.

Original authors: Patricia Donald and Lewis Jett