Key Sources of Plant Diseases in Nursery Production

Infested Soil

Pathogens in soil include Cylindrocladium, Pythium, Phytophthora, Fusarium, Rhizoctonia, Thielaviopsis, crown gall bacteria, and nematodes. Pythium also occurs in sand and peat. Use only pathogen-free mixes (treated or certified soilless), stored on clean surfaces, moved with clean tools, placed in clean containers.

Soil from surrounding areas can contaminate containers via wind, feet, dirty machinery/tools. Clean and disinfect tools, hose ends, etc. Place containers on a porous plastic sheet covered with 4–6 inches of coarse gravel or stone to separate from underlying soil, reduce puddles, and minimize splash.

Plant Debris

Pathogens survive dormantly in dead tissues: Erwinia, Pseudomonas, Botrytis, Phytophthora, Thielaviopsis, Rhizoctonia, Cylindrocladium, Phomopsis, Botryosphaeria, Sphaeropsis, Verticillium, Cytospora, Nectria, foliar nematodes, and tobacco mosaic virus. Remove infested debris to prevent recurrence.

Plants

Viruses (tomato spotted wilt, cucumber mosaic) and rusts (pine gall, cedar-quince) require living tissue. Plants in and around the nursery act as reservoirs. Remove alternate hosts (e.g., wild apples/hawthorns near juniper crops; wild junipers near crabapple/quince/hawthorn crops).

Purchased plants may already be infected (Agrobacterium, Phytophthora). Know your supplier; they are legally obligated to sell healthy plants. Change suppliers if infected material is received. This is critical for vegetatively propagated crops where stock plant diseases (vascular wilts, viruses, foliar nematodes) transfer to cuttings. Propagators must control stock plant diseases. Inspect incoming material immediately; delayed inspection obscures the source. Train employees as scouts to spot problems early.

Water

Phytophthora and Pythium cause damping-off, root/stem rots, cutting rots, and top dieback. Surface waters contain Pythium in sediment; some also have Phytophthora in sediment and free in water seasonally. Runoff can contaminate wells. Avoid pumping bottom sediment. If Phytophthora is a recurring problem, test water sources, especially if recycling water.

Air

Powdery mildew and rust spores travel long distances on air currents from gardens and forests. Eliminate non-crop pathogen hosts within and around the nursery.

Conclusion

These are the first sources to suspect. Every nursery manager should identify pathogen sources for each crop and plan to eliminate them. Prevention saves money by reducing losses and chemical costs.