Watch for diseases in propagation

No matter where you are in the propagation/production cycle, disease prevention or control is your responsibility. Recognizing where a disease starts is the first step in any disease-control strategy. You need to be able to identify the problem as early as possible.

Early preventive strategies can be the least costly with the maximum benefit.

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Bulbs and seeds. Some pathogens are naturally occurring on bulbs and seeds that became infected from the parent crop. Nearly all bulbs or seeds are produced under field conditions where they are most easily contaminated. They must remain protected for the entire crop life, which makes prevention a serious challenge.

Seed can also become contaminated during flowering since seed-borne pathogens like Alternaria spp. are easily spread by wind. Pseudomonas and Xanthomonas can be spread by rainfall or overhead irrigation. Finally, using fungicides and bactericides during seed production phases may affect seed formation or quality and is not always successful.

Cuttings and liners. Cuttings can be contaminated during stock plant production or shipping. Some pathogens are present systemically in stock plants without showing symptoms. Cuttings can be contaminated during harvesting. Viruses spread this way as well.

Others cuttings can be contaminated with pathogens during shipping. These include opportunistic pathogens like Botrytis, Erwinia and Myrothecium. It is common to see New Guinea impatiens with myrothecium leaf spot present on the unrooted cuttings. Liners of spathiphyllum and syngonium can also develop myrothecium petiole rot during shipping.

Propagation contamination. Some diseases start in propagation when benches are not thoroughly cleaned between crops or when flats are reused without effective disinfesting. Rhizoctonia cutting rot and damping-off is most commonly spread this way since the fungus does not produce spores that can easily be splashed around. In the past year, we have also found evidence of Pythium in new potting media.

Control strategies

Bulbs and seeds. If you can’t find pathogen-free bulbs or seed, you can control some diseases through modification of the environment and use of bactericides and fungicides. Many diseases will not develop if conditions don’t favor them.

For botrytis, the disease can be controlled through management of humidity around the plants with spacing, horizontal-airflow fans and venting and heating at sunset.

Even soil-borne disease pathogens like Pythium are hard to maintain in control experiments if the plants are treated too well. Good cultural practices include using a well-draining potting medium, providing the plants with optimal light levels and fertilizer, maintaining temperatures and avoiding excess irrigation.

Treating with a preventive fungicide or bactericide can be very effective. These products can damage the plants without benefit. Do everything you can culturally to reduce disease levels before resorting to chemicals.

Sometimes, seed can be cleaned of pathogens using chlorine-based products. The treated seed must be dried thoroughly before storage and tests should be conducted to make sure the treatment does not reduce germination. This control technique doesn’t work on primed and coated seeds.

Cuttings and liners. If you are a cutting or plug propagator, it is critical that you determine and maintain the right conditions during shipment. Use temperature recorders to monitor each shipment if you suspect any problems. For plugs, a fungicide treatment the day before shipping may be a good way to prevent downy mildew.

Cuttings can be contaminated during shipping and usually have to be treated with a fungicide. In the case of Erwinia, the pathogen can sometimes be controlled by minimizing water during the rooting process.

Preventing propagation contamination. If you have problems with Rhizoctonia cutting rot, make sure your cutting stock plants are disease free. Always use new potting media and flats.

If necessary, you can treat used flats with steam or with a soak in a disinfestant like quaternary ammonium or bleach. Cleaning before soaking is absolutely critical or the disinfestant may be only partially successful. Treatment of flats for Pythium contamination has been very effective using a 30-minute dip of GreenShield at 0.5-1 ounce per gallon following washing. Even an overnight soak without cleaning was not 100-percent effective.

Contaminated potting media are starting to be a major concern. It is not surprising to find Pythium or Fusarium in potting media that are recycled or when a medium was prepared on site and sanitary conditions were not followed. However, I did not expect to see Pythium contamination in new, unused bagged potting media.

Some plants affected by disease during propagation

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- Ann Chase