How to Manage Pests
Pests in Gardens and Landscapes
In the field, garden, or planter box, seedlings often fail to come up, or die soon after they have emerged from the soil. Seeds may rot before they germinate, shoots may be decayed before they emerge, or stems of seedlings may be attacked near the soil line, causing young plants to collapse. These diseases often are collectively referred to as “damping-off,” and may be caused by a number of soil-inhabiting pathogens.
Species of the soil organism Pythium are most often responsible for damping-off, but several other pathogens, including species of Rhizoctonia, Fusarium, and Phytophthora, can also cause decay. Decay is most likely to occur when old seeds or seed pieces are planted in cold wet soil, and is further increased by poor soil drainage, the use of green compost, and planting too deeply.
The first evidence of damping-off or seed piece decay (as in potatoes) is the failure of some plants to emerge. If seeds are attacked before they germinate, they become soft and mushy, turn dark brown, and decay. They may have a layer of soil clinging to them when they are dug up because the soil is interwoven with fine, threadlike fungus growth. Germinating seedlings shrivel and may darken. If seedlings are attacked after they emerge, stem tissue near the soil line is decayed and weakened, usually causing plants to topple and die. When only roots are decayed, plants may continue standing but remain stunted, wilt and eventually die. As seedlings get older, they become less susceptible to damping-off pathogens.
Garden pests such as cutworms, earwigs, flea beetles, snails and slugs, and root maggots may also damage seedlings in the garden. It’s important to distinguish the damage done by pests from damping-off injury.
The pathogens that cause damping-off and seed piece decay are present in virtually all soils. They survive on dead organic matter and also produce spores or other structures that survive for long periods of time. The young tissue of emerging seedlings is least resistant to infection when plants are growing slowly in cold, wet soil. Vigorously growing seedlings are fairly resistant to infection.
Damping-off pathogens can be divided into two main groups. The “true fungi” include Rhizoctonia, Fusarium, and Thielaviopsis genera. A second group, previously considered fungi but now placed in a separate classification called oomycetes, includes the Pythium and Phytophthora genera.
Damping-off is controlled primarily through good sanitation, high quality planting material, and proper cultural and environmental controls. Damping-off is worse when soil is wet or compacted. Prepare planting beds so that the soil has good drainage. Drainage can be improved by using raised beds and soil amendments such as redwood shavings, peat moss, or fir bark. Use only well-decomposed compost. The overly moist environment of green compost will encourage a damping-off problem. Use aerobic (well aerated) composting procedures to reduce the population of disease-causing pathogens in the compost. Composted hardwood bark has been reported to reduce damping-off.
Plant when temperatures are favorable for rapid seedling growth. Shallow planting will speed up germination outdoors if conditions are marginal. Planting too deeply can delay germination and emergence of the seedling and increase damping-off problems. If you want to start seedlings before temperatures are favorable, start them in the greenhouse or other protected areas and transplant them into the field when temperatures get warmer. Do not transplant into cold, wet soil. Use only the highest quality seed available. Use light sprinkler irrigations to encourage germination and emergence, and do not overwater. After plants emerge, thin them so that there is good air circulation among the plants. Avoid putting on too much nitrogen fertilizer. Avoid planting the same crops in the same place year after year.
Potatoes are a special case. If growing potatoes, plant seed pieces in moist soil that is at least 50° F, and start them at a time of year when irrigations will not be necessary before sprouts emerge from the ground.
Sanitation is important because spores of the organisms that cause damping-off can survive in dust, planting medium, or soil particles in flats and pots. To reduce survival of the pathogens, remove and discard diseased plants and sterilize containers.
If starting plants indoors, in cold frames, or in greenhouses, plant seeds in steam treated soil or pasteurized potting mix. You can purchase treated potting soil or prepare your own. Soil mix must be held at 140° F for at least 30 minutes. Soil can be heated in a conventional or microwave oven in a paper bag, by raising the temperature with boiling water, or by placing moist soil in a clear plastic bag in direct sunlight. Use a soil thermometer to make sure soil reaches the proper temperature for the correct time.
For outdoor planting beds, in the warmer regions of California, soil solarization during fallow periods can reduce pathogens. To solarize, place clear plastic tarps over moist, bare soil for 4 to 6 weeks during the hottest part of the year.
Elmore, C. L., J. J. Stapleton, C. E. Bell, J. E. DeVay. 1997. Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds. Oakland: Univ. Calif. Agric. Nat. Res. Publication 21377.
Flint, M. L. 1998. Pests of the Garden and Small Farm: A Grower’s Guide to Using Less Pesticide. Oakland: Univ. Calif. Agric. Nat. Res. Publication 3332.
Author: E. J. Perry, UC Cooperative Extension, Stanislaus County
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