Plant problems can be prevented with appropriate preparation and attention. The American Horticultural Society’s New Encyclopedia of Gardening Techniques (Mitchell Beazley, 2009) gives a detailed look into planting from preparation to harvest. Learn the basics of plant disease and prevention in this excerpt taken from Chapter 4, “Growing Vegetables & Herbs.”
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Most plant diseases affecting crops are fungal or viral. Airborne fungal infections are spread by spores, so good spacing and airflow help to reduce incidence, but some are spread in the soil and are harder to combat. Viruses are mainly spread by handling, or by sap-sucking insects.
This fungus is common on many brassicas. White, chalky pustules develop on the undersides of leaves, and the upper surfaces are sometimes distorted and discolored. It is unsightly, but not serious. Remove affected leaves, and reduce incidence by spacing plants well and using rotation.
Onion white rot
Roots and basal tissues develop white, fluffy growth and rot, sometimes causing plants to fall over, and leaves to yellow and die. Dig up and destroy affected plants. The fungus produces black spores that can survive in soil for 15 years. Grow onions elsewhere or replace the soil: do not spread it.
Onion downy mildew
Onion leaves wither and collapse. If humid, an off-white mold develops. The bulbs do not store well. Destroy affected plants and avoid growing onions on the site for five years. Control weeds to encourage good airflow.
Several fungi and bacteria cause brown spots, particularly on older leaves and in wet seasons. Diseased tissue may fall out to leave shot holes. Destroy affected leaves and if necessary remove alternate plants to improve airflow.
This can affect all the brassica family. Plants are stunted and leaves may wilt on hot days, recovering overnight. On lifting, the roots are thickened and distorted. This is a slime mold, usually introduced on brought-in seedlings, and possibly manure from cattle fed on diseased plants. It can survive in soil for 20 years, and is worst on acid, wet soils. Liming and improving drainage will help, as will raising seedlings in pots and planting out when larger. No chemicals are available, but there are resistant varieties.
This fungus forms elongated pustules of orange spores on leeks, onions, garlic, and shallots. Severe infections can cause dying of leaves and small bulbs. Late season foliage is normally healthy. Applying sulfate of potassium is claimed to help, as will clearing infected material and crop debris, good drainage, wide spacing, and using a long rotation. Resistant varieties are available.
Dark brown pustules appear on leaves, stems, and pods of pole and bush beans. The white, cluster-cup stage of the fungus may develop later in the season. Destroy affected tissue when seen.
This disease affects many vegetables including beans, potato, tomato, and celery. Space plants to encourage air circulation and avoid wetting foliage. Late planting of potatoes sometimes helps limit infection.
Common scab: This is caused by a bacterium and scabby spots with irregular edges develop on the skin. Although unsightly, the damage is not very serious. Scab occurs on light soils lacking organic matter and is worst in dry years. Dig in compost or other organic matter and ensure a regular supply of water when the tubers are forming, from two to three weeks after emergence, continuing for at least four weeks. Liming can encourage common scab, so avoid growing potatoes on ground limed for a previous brassica crop. Resistant varieties are available.
Powdery scab: This fungal disease causes irregular, brown depressions with raised edges, containing dusty masses of spores. Badly affected tubers are swollen and worthless. It is worst on wet soils and in wet years. Plant tubers that have as low a level of infection as possible. Some varieties are more resistant than others.
Foot and stem rots
Several fungi can cause these rots, and some vegetables such as tomatoes and those of the cucumber family are prone to infections. The roots or stem base rot, and the plant collapses. Irregular watering or a poor root system exacerbate the problem. If caught early, foliar feeding may encourage new root production. Destroy severely affected plants and replace the soil around the roots.
Late blight of potatoes and tomatoes is caused by Phytophthora infestans. Brown dead patches appear at the leaf tips and enlarge to kill the leaf. In dry weather the infection may slow, but in wet weather it spreads rapidly.
Potato blight: Spores can be washed onto the ground where they infect the tubers. The rot is a hard, reddish brown patch that extends into the tuber. Secondary bacteria often infect these wounds to cause a slimy soft rot. Affected tubers will not store.
The airborne spores can infect plants over wide ranges. Foliage must be sprayed with a protectant fungicide before blight appears. If blight arrives late in the season, it is best to remove the stems and leaves so that the tubers do not get infected.
Some vegetables, such as the cucumber family and peas, are particularly susceptible to powdery mildew infection, which can spread quickly under dry conditions but causes significant damage when humidity is high and air circulation is poor. Use resistant varieties when available.
This common disease of mint and related plants turns stems and leaves pale and distorted before erupting as masses of orange pustules, which turn black. Leaf tissue dies and plants are defoliated. The fungus is perennial in garden mint, but spores also overwinter in the soil. Apply an appropriate fungicide.
Viruses on tomatoes
Typically, viruses cause mottling and distortion of leaves, stunting, and poor fruit yield, but some symptoms are very similar to those caused by herbicide exposure or cold damage. Tobacco mosaic virus (T.M.V.) is highly contagious and serious. Fruit may not set and young fruit are ‘bronzed’ or streaked. Destroy affected plants immediately; extensive spread may have occurred but not yet be obvious. Clean tools and hands well, and control pests. Some varieties are marketed as resistant to T.M.V.
This virus disease causes foliage to become thick and leathery and the leaves to curl upward. It attacks a wide range of crops including beans, brassicas, and squashes as well as beets, chard and spinach, carrots, and celery. It is especially prevalent west of the Rockies. Curly top is spread by leafhoppers, so use row covers to prevent the leafhoppers attacking the plants.
Smut on corn
This disease is common in warmer regions, and seen in hotter summers elsewhere. Plants may be stunted. Cobs, flower tassels, stems, and occasionally leaves develop dramatic ashen malformations, from which a dark spore mass later erupts to cause new infections or survive in crop debris or soil. Destroy all infected material and grow no corn on the site for five years.
A paperback edition of New Encyclopedia of Gardening Techniques will be released in April 2013.
This excerpt has been reprinted with permission from New Encyclopedia of Gardening Techniques by The American Horticultural Society, published by Mitchell Beazley, 2009.