Sources Of Plant Disease In Nurseries
After reviewing this article, make an inventory of the practices you employ in your operation and note which make your crops vulnerable to a disease problem as well as those which help avoid diseases.
Many plant pathogens can be found in soil. Fungi such as Cylindrocladium, Pythium, Phytophthora, Fusarium, Rhizoctonia, and Thielaviopsis, crown gall bacteria (Agrobacterium) and most nematodes reside in the soil. Pythium species can be found in sand and peat as well. When containerized nursery crops are potted in a mix containing these pathogens, the pathogens are stimulated into activity by nutrients that leak from the plants' roots and disease may begin. Therefore, the potting mix must be free of pathogens before planting. A potting mix that has been treated to kill plant pathogens or a soilless mix purchased with the assurance of being free of pathogens should be used so that unwanted organisms are kept out. It should be stored on a clean surface, moved with clean implements to a clean potting area, and placed in clean containers.
No matter how careful a grower is, disease caused by soilborne pathogens still can occur in container production. Besides the potting mix, soil from the surrounding area can be moved into the containers by the wind, by workers' and pets' feet, and by using dirty machinery and tools. Care must be taken to avoid getting contaminated soil into the potting mix. Tools, hose ends and other things that have the potential of moving pathogen-containing soil into a pathogen-free potting mix must be thoroughly cleaned and disinfested. Containers are often placed on the ground and not separated from the underlying soil. It is best to first put down a sheet of porous plastic and then cover the sheet with 4 to 6 inches of coarse gravel or small stones. By placing the containers on this surface, they are away from underlying and possibly contaminated soil. This surface also greatly reduces puddles and splashing of irrigation water.
Most plant pathogens have a stage in their life histories that can rest in a dormant state and survive periods of time when temperatures are extreme or moisture is not sufficient for growth. Some pathogens have evolved a strategy of becoming dormant in the dead leaves, stems, branches, and roots where they previously caused disease. Inside those tissues they are protected from the hostile environments of the soil and air and are away from competition with other organisms in the soil and air. They have at hand a ready supply of nutrients for when conditions become favorable again. Bacteria such as Erwinia and Pseudomonas, fungi such as Botrytis, Phytophthora, Thielaviopsis, Rhizoctonia, Cylindrocladium, Phomopsis, Botryosphaeria, Sphaeropsis, Verticillium, Cytospora, and Nectria, foliar nematodes (Aphelenchoides), and tobacco mosaic virus, survive for months in plant debris. A disease may recur if infested debris is left in the nursery where it may come in contact with the crop.
Some pathogens must have living plant tissues in order to grow, reproduce, and survive. Most viruses like tomato spotted wilt and cucumber mosaic only survive in living plant cells. Rusts, such as pine gall rust and cedar-quince rust, must pass from living plants to other living plants or they die. Thus, plants in and around the nursery act as reservoirs of pathogens and should be under strict disease control.
Non-crop plants (weeds) fall under this heading. If junipers are the crop, then wild apples and hawthorns should be removed from in and around the nursery because they are alternate hosts of cedar-apple, -quince, and -hawthorn rusts. Likewise, if crabapples, quinces, or hawthorns are the crops, then wild junipers should be removed from in and around the nursery.
Recognize that the plants you buy from a supplier to grow on in your operation may already be infected with pathogens including Agrobacterium and Phytophthora . Get to know the operation from which you buy plants. It is their responsibility and legal obligation to sell only healthy plants. If you find that you are receiving infected seedlings or young transplants, change suppliers. This is especially true if you purchase vegetatively propagated crops. Any disease affecting stock plants is likely to be found on cuttings taken from those plants, particularly if the pathogens reside inside the plant. Vascular wilt diseases such Verticillium wilt, virus diseases, foliar nematodes, all will accompany cuttings if stock plants are infected. The propagator must assume responsibility for strictly controlling diseases on stock plants so that these diseases are not sold to you. The Department of Agriculture inspectors are of great assistance in inspecting plants. However, you must also inspect purchased material carefully as soon after arrival as possible. If inspection is put off until the plants have been in the nursery for several months, it is not possible to determine whether the pathogens accompanied the plants or moved to them from sources within your operation. Train your employees to look for problems. Workers who handle a large number of plants is most likely to first notice a problem. Encourage them to be your 'scouts'.
Phytophthora and Pythium, which can cause damping-off, root and stem rots, cutting rots, and top diebacks are probably the main pathogens that can be spread in the nursery in water. Surface water supplies such as lakes, ponds, rivers, and streams contain Pythium in the bottom sediment. Some also contain Phytophthora in the sediment as well as free in the water at certain times of the year. Note that run-off can carry these fungi from into wells. Care should be taken to avoid pumping bottom sediment from water supplies into the irrigation system. If Phytophthora is an ongoing problem in your nursery, your water source should be tested for that fungus. This is particularly true if you recycle water within the nursery.
The spores of powdery mildew fungi, rust fungi, and others can be carried by air currents for long distances outdoors. Infected plants in nearby gardens and forests can supply enormous numbers of spores. Thus, even if great care is taken to eliminate other sources of pathogens, the air we breath may carry certain disease-causing organisms into the nursery. This is another reason why non-crop plants that harbor pathogens of crops plants should be eliminated from within and around the nursery.
There are other sources but these are the first to suspect when trying to answer the question "Where did the disease begin?" Every nursery manager should be aware of the sources of pathogens for each crop grown in their particular operation and should plan to eliminate those sources of disease to the greatest extent possible. Money can be saved by not losing plants during production and by minimizing the expense of purchasing and applying disease control chemicals.
Prepared by Gary W. Moorman, Professor of Plant Pathology
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