Infested Soil
Many plant pathogens can be found in soil. This includes:
Fungi such as Fusarium, Rhizoctonia, and Thielaviopsis
Oomycetes such as Pythium, Phytophthora
Bacteria such as crown gall bacteria (Agrobacterium)
Many types of nematodes
Pythium species are found in sand and peat as well. When greenhouse 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 handled as if it were food and kept free of unwanted organisms. It should be stored on a clean surface, moved with clean implements to a clean potting bench, and placed in clean pots or flats.
No matter how careful a grower is, diseases caused by soilborne pathogens still can occur. Besides the potting mix, soil is found in many other places in the greenhouse. Soil is usually under benches, in aisles, and in the benches in older greenhouses. Soil is brought into the greenhouse on workers' and pets' feet, on machinery used to move materials into the greenhouse, and on crates, flats, and boxes stored outdoors unprotected on the ground. Care must be taken to avoid getting this soil that may be contaminated into the potting mix. Tools, hose ends, and other things that have the potential to move pathogen-containing soil into a pathogen-free potting mix must be thoroughly cleaned and disinfected. If old benches are full of soil, the soil should be sterilized or covered with clean plastic sheets to separate it from the potted plants placed on the bench.
Debris from Previous Crops
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, 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 when conditions become favorable again. Bacteria such as Dickeya dadantii (formerly Erwinia chrysanthemi), fungi such as Botrytis, oomycetes such as Pythium, foliar nematodes (Aphelenchoides), and tobacco mosaic virus survive for months in plant debris. A disease may recur if infested debris is left in the greenhouse where it may come in contact with the next crop.
Plants Kept All Year
Some pathogens must have living plant tissues in order to grow, reproduce, and survive. Viruses like impatiens necrotic spot and cucumber mosaic only survive in living plant cells. Rusts, such as geranium or fuchsia rust, must pass from living plants to other living plants or they die within weeks. Powdery mildew fungi may be on grape ivies, begonias, roses, and African violets unnoticed or at a level of severity thought insignificant until they later explode into activity. Similarly, Botrytis on geraniums can usually be found on branch stubs and fading leaves and flowers most of the year. When light, humidity, and temperature conditions turn in favor of these pathogens, the disease can seem to appear and spread rapidly when, in fact, the problem has been building for some time. Thus, plants kept in the greenhouse all year act as reservoirs of pathogens and should be under strict disease control.
Weeds (especially bittercress and oxalis) fall under this heading as do plants such as Tradescantia and English ivy that are allowed to escape and grow under benches. Plants found all year in the greenhouse not only harbor pathogens, they are also excellent havens for the thrips, whiteflies, and aphids that can spread diseases.
Water
Phytophthora and Pythium, which can cause root and stem rot, and cutting rots are probably the main pathogens that can be brought into the greenhouse in water.
Surface water such as lakes, ponds, rivers, and streams contain Pythium or Phytophthora. Run-off can carry these oomycete pathogens and various fungi from the soil into wells. Care should be taken to avoid pumping bottom sediment from water supplies into the greenhouse irrigation system. Pythium and Phytophthora are major problems in hydroponic systems and can become important in ebb and flow systems.
Conclusion
These are the major sources of living things that cause diseases in greenhouse crops. There are other sources but these are the first to suspect when trying to answer the question "Where did the disease begin?" Every greenhouse 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. Money can be saved by not losing plants during production and by minimizing the expense of purchasing and applying disease-control chemicals.
