Seasonal disease control

Don’t close the greenhouse door on plant health this winter. Watch for certain diseases this time of year.

Botrytis crown rot on primrose
Photos courtesy of A. R. Chase

As fall slowly turns into winter throughout most of the country, gray mold (Botrytis) is just getting started. The diseases that occur in each season are somewhat based on what plants we grow, but also affected dramatically by the weather. Plants grown out of their optimal range of temperature, day length and humidity can be stressed and weakened, resulting in more disease problems than they would normally experience. Greenhouses in many parts of the country are enclosed, making air exchange less effective while growers seek to retain heat and continue good plant growth. Lower light levels also affect diseases as the plants grow less vigorously, sometimes allowing diseases to get ahead. This can be especially true of diseases like those caused by Fusarium, which may have been around for months but waited for stressed plants to finally take off. Plants also need less water during the winter and unless growers are diligent about checking irrigation rates, they may end up over-watering and starting a Pythium root rot epidemic.

Botrytis is just getting going

Botrytis blight is very common on dahlia, fuchsia, geranium, cyclamen, exacum, poinsettia, pansy and lisianthus, but all ornamentals can be affected by this non-discriminating fungus. In winter, Botrytis cinerea is expected to cause losses due to the lower temperatures, which favor its development. Watch for tip blights and stem rot more often than petal and leaf spots. This can be due to the limited use of overhead irrigation during the winter. If you cannot use heating and venting to reduce humidity overnight, then a fungicide may be needed.

There are many different fungicides that can be very effective in preventing Botrytis — prevention is always more effective than attempting to cure a Botrytis blight situation. Trials show that Botrytis is best controlled with these fungicides:

  • chlorothalonil (Daconil formulations-do not use on open flowers) FRAC M5
  • fenhexamid (Decree-best fungicide for killing Botrytis spores) FRAC 17
  • fludioxinil (Medallion, Palladium) FRAC 12 or FRAC 9/12
  • iprodione (Chipco 26019, Chipco 26GT) FRAC 2
  • strobilurin/SDHI (Broadform, Mural, Pageant Intrinsic) FRAC 7/11

Choose two or three products in different FRAC groups that suit your needs and rotate between them. Botrytis resistance has been reported in many of the best fungicides. Be especially careful to read fungicide labels and use all products according to their labels only.

Pythium is always around

Due to the critical timing of many crops (including spring bulbs), it usually pays to apply fungicides for Pythium preventatively. Some of the oldest fungicides for Pythium contain etridiazole (Banrot, Terrazole or Truban-FRAC 14) and they are still used in some portions of the country. Mefenoxam (Subdue MAXX-FRAC 4) however, is perhaps the most common fungicide for Pythium prevention. Segway O (cyazofamid) is also very effective on Pythium root rot and is in a separate FRAC group — 21. Other fungicides may be registered for Pythium root rot but are not as consistently effective.

Fusarium wilt on cyclamen
Pythium damping-off on alyssum

In other cases, growers use biologicals like RootShield Plus (Trichoderma harzianum) and Obtego (Trichoderma asperellum and T. gamsii) with excellent results when used preventatively. In addition, some trials have demonstrated improved rooting even without Pythium present with use of Obtego.

As always, rotate between fungicides to prevent development of resistance. Usage of etridiazole for the past 40+ years has shown no resistance in Pythium populations. In contrast, some Pythium populations have become resistant to mefenoxam (Subdue MAXX).

Fusarium diseases ramp up

Symptoms of Fusarium include leaf spots, stem and crown rot, root rot, and wilt. Stem rot is usually somewhat mushy and often mistaken for Erwinia infections, but it can be dry too. Wilt can be accompanied by brown streaks in the xylem and above ground symptoms are often one-sided. Yellowing can occur, and wilt of a single branch or section of an infected plant can occur while the rest of the plant looks healthy.

Fusarium wilt diseases react to nitrogen and the effects are often indirect through a plant response and not a direct fungus response. Research consistently shows that managing a nitrogen source and maintaining soil/medium pH are the more effective than fungicides. When crops are fertilized with lower levels of nitrogen derived from nitrate, rather than ammonium, severity of Fusarium wilt is minimized.

Fusarium diseases are especially hard to stop, even when fungicides are used preventatively. Traditionally, we have relied on the use of thiophanate methyl (FRAC 1- 3336, OHP-6672) and iprodione (Chipco 26 products, FRAC-2). In the past 20 years, we saw the introduction of triflumizole (Terraguard, FRAC 3), fludioxonil (Medallion, FRAC 12), and the strobilurins (like Heritage-FRAC 11). Most recently, we were introduced to Postiva, which is a combination of the FRAC 3 and FRAC 7 active ingredients. It has consistently resulted in the highest levels of Fusarium stem rot, root rot, and wilt intervals that I have seen in my career.

Keys to Fusarium control

  • Use only healthy cuttings, bulbs, seedlings, or liners — if any are available.
  • Discard infected plants as soon as they are found.
  • Minimize plant stress and avoid ammoniacal nitrogen on susceptible crops.
  • Rotate fungicides between different FRAC groups.

Conclusions

The list of diseases that attack ornamentals is very long and unfortunately grows with every passing year. The list will only grow as we seek out new crops that have new insects and new diseases. One of the keys to making ornamental production profitable is timely disease control. Using products to prevent all diseases would be too costly and undoubtedly counter-productive, as they can slow crop times in certain cases. It is therefore important to know when to watch for specific diseases and use cultural prevention whenever possible.

About the author: A. R. Chase is founder of Chase Agricultural Consulting; archase@chaseresearch.net; (530) 391-3068.

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