Common Sources of Plant Diseases in Greenhouse Operations

Infested Soil

Many pathogens—Pythium, Phytophthora, Fusarium, Rhizoctonia, Thielaviopsis, crown gall bacteria (Agrobacterium), and nematodes—reside in soil. Pythium also occurs in sand and peat. When crops are potted in contaminated mixes, root exudates stimulate pathogen activity. Use only pathogen-free mixes (heat-treated or certified soilless) and handle them like food: store on clean surfaces, move with clean tools, use clean pots/flats.

Even with care, soilborne diseases can arise. Soil under benches, in aisles, and on older greenhouse benches harbors pathogens. Workers' feet, pets, machinery, and outdoor-stored crates introduce contaminated soil. Clean and disinfect tools, hose ends, and any items contacting the mix. If old benches hold soil, sterilize or cover with clean plastic to separate from pots.

Debris from Previous Crops

Most pathogens have dormant survival stages in dead leaves, stems, and roots. Erwinia chrysanthemi, Botrytis, Pythium, foliar nematodes (Aphelenchoides), and tobacco mosaic virus survive months in debris. Remove all crop debris to prevent carryover to the next crop.

Plants Kept Year-Round

Some pathogens require living tissue. Viruses (impatiens necrotic spot, cucumber mosaic) survive only in living cells. Rusts (geranium, fuchsia) die without living hosts within weeks. Powdery mildew and Botrytis persist on grape ivy, begonias, roses, African violets, and geraniums, often unnoticed until favorable conditions trigger outbreaks. Year-round plants act as pathogen reservoirs and must be under strict disease control. Weeds (bittercress, oxalis) and escaped plants (Tradescantia, English ivy) also harbor pathogens and insect vectors (thrips, whiteflies, aphids).

Vegetatively Propagated Crops

Cuttings may carry diseases from stock plants. Vascular wilts (Ralstonia, bacterial blight of geranium, Fusarium wilt of chrysanthemum, Verticillium wilt of impatiens), viruses (dasheen mosaic, INSV, tomato ringspot), and foliar nematodes can all come on cuttings. Propagators must rigorously control diseases on stock. Buyers should inspect incoming material immediately; delayed inspection makes it impossible to distinguish source vs. greenhouse-acquired infection.

Culture and virus indexing reduce but do not eliminate risk. Indexed plants are tested only for specific pathogens; others (e.g., Botrytis) may be present. As indexed plants remain in the greenhouse, re-infection probability increases.

Water

Phytophthora and Pythium are the main waterborne pathogens. Surface waters (lakes, ponds, rivers) contain them in sediments. Runoff can contaminate wells. Avoid pumping bottom sediment. These pathogens are major problems in hydroponics and can threaten ebb-and-flow systems.

Air

Spores of powdery mildews, Botrytis, rusts, Alternaria, etc., enter via air currents from outdoor plants and weeds. Maintain greenhouse conditions that inhibit pathogen activity and eliminate non-crop pathogen sources around the structure. Use caution with herbicides near greenhouses—vapors can damage crops. Since most greenhouse crops are dicots, a grassy lawn around the greenhouse can reduce spore influx (few pathogens attack both monocots and dicots).

Conclusion

These are the primary disease sources. Every greenhouse manager should identify pathogen sources for each crop and plan to eliminate them. Prevention saves money by reducing plant losses and chemical control costs.