Potted Flowering Crops — What Are Your Worst Enemies?
This is the final article in the “worst enemies” series and will cover some of the most important flowering crops from hydrangea, cyclamen and geranium to Easter lily, poinsettia
and orchids.
The characteristics of potted flowering crops that affect their diseases are:
- propagated by division, cutting or bulb
- grown to be sold in flower
- very high value
- grown for specific market windows
- grown in mass (monoculture is common)
- relatively long production time
The most common problems of potted flowering crops include : Fusarium wilt and crown rot, root rot, Botrytis blight, powdery mildew and Pythium root rot.
FusariumDiseases
The Fusarium fungi are opportunistic and survive well in the potting medium, in soil under benches and even on bench surfaces as a saprophyte without a susceptible plant host. Those that can cause crown rot (and pseudobulb rot on orchids) are very broad in their host range with no real specialization shown. In contrast, the Fusarium responsible for wilt disease are very specific to a single genus or family of plant.
Fusarium wilt is most common during production, often showing the first symptoms as crops near finishing. Fusarium oxysporum f.sp. cyclamenis causes wilt of cyclamen often showing up midway through production. The outcome of Fusarium wilt on cyclamen is death as even the most effective fungicides cannot stop the disease once infection occurs. Avoid water stress since this often is the trigger for an infection. For orchids, a long ocean voyage from their homeland can turn excellent looking liners or pre-finished material into mush a few weeks after arrival.
Fusarium leaf rot and pseudobulb rot on many orchids is very common. The disease causes very large, wet- looking rotted areas which are often confused with soft rot caused by Pectobacterium (=Erwinia).
Never use infected cuttings or dip them into a fungicide bath (this spreads spores very effectively contaminating the entire batch). Do not overwater or use a poorly draining potting medium. Do not plant in ground beds that have become contaminated from a previous crop. Don’t use pots or flats without a thorough cleaning and disinfesting and never reuse potting media. Avoid using ammoniacal nitrogen or too much nitrogen on plants that are susceptible to Fusarium wilt.
The best fungicides are strobilurins (FRAC 11) especially azoxystrobin, triflumizole (FRAC 3) and fludioxonil (FRAC 12). Alternating drenches or sprenches of two of these three FRAC groups is the best way to delay resistance in the fungus to a particular active ingredient.
Botrytis Blight
Gray mold (Botrytis cinerea) is very common on dahlia, fuchsia, geranium, cyclamen, exacum, poinsettia, pansy and lisianthus, but really all ornamentals can be affected by this non-discriminating fungus. On some crops, Botrytis cinerea infections are confined to flower spots, while on others, cutting rot, stem rot and leaf spot also occur.
High humidity and more than six hours of leaf wetness and temperatures around 50 to 75° F are most important for an outbreak of Botrytis. Poor air circulation also promotes this and other foliar diseases, since it means leaves will stay wet longer and relative humidity around the plant is higher. Damage to the plant (phytotoxicity and mechanical wounding) promote a Botrytis outbreak. We have seen copper fungicide applications actually cause more Botrytis damage. This happens when they fail to dry quickly since they can cause phytotoxicity. Unfortunately, spraying plants more than once a week also can lead to an outbreak, since this adds a lot of unnecessary water to the canopy.
There are quite a few fungicides that are very effective against Botrytis. Some of the most effective include : fludioxonil (FRAC 12) and in a premix with cyprodinil (FRAC 9), iprodione (FRAC 2) and a premix of pyraclostrobin (FRAC 11) and boscalid (FRAC 7).
Powdery Mildew
Fall is the right time for powdery mildew with warm days and cool nights. Condensation that occurs overnight promotes powdery mildew. Watch roses, begonia, gerbera daisy, hydrangea, African violet and poinsettia for signs of this common disease. In a greenhouse, controlling humidity can go a long way in preventing powdery mildew from starting.
You can use sterol inhibitors (FRAC 3 myclobutanil, triademifon and triflumizole) as well as strobilurins (FRAC 11 azoxystrobin, trifloxystrobinm and pyraclostrobin). Rotate between these two FRAC groups and others for best results. Early work on poinsettias with triflumizole actually showed a single application early in the crop cycle protected poinsettias from powdery mildew until finishing. While it is often possible to stop an outbreak of powdery mildew, it is not the case with hydrangeas which can be very hard to cure powdery mildew on once infected. Think preventatively!
Pythium Root Rot
The most common disease of poinsettias in the fall is Pythium root rot. If the cuttings make it through rooting without a Rhizoctonia stem rot or Erwinia blight outbreak, they can sometimes develop Pythium root rot later in their life. Symptoms include wilt, even when the potting medium is wet, slow growth, poor color (especially yellow lower leaves), weakened stems and of course root rot. Roots appear gray or water- soaked and disintegrate easily when handled. They often show the classic “Pythium” characteristics of retaining their central core while losing the outer cortex of the roots.
Due to the critical timing of some crops (like poinsettias and Easter lilies), it usually pays to apply fungicides preventatively. Etridiazole (Banrot, Terrazole or Truban) is used in some portions of the country while in others Subdue MAXX or Aliette is the prevention of choice. In still others, RootShield Plus (Trichoderma harzianum T22) is used exclusively with excellent results. Poinsettia is one of the most responsive crops to the biological protection afforded by Trichoderma.
Be sure to rotate between fungicides to prevent development of resistance especially with Pythium spp. Pythium populations in some greenhouses have become resistant to mefenoxam (Subdue MAXX) which does not appear to go away even when mefenoxam has not been used for more than 20 years. If you have relied on Subdue MAXX alone and are not happy with the results, you may have a resistant Pythium. Using more will not overcome most mefenoxam resistance. Lab tests can confirm this situation.
Finally, phosphonates (phos acid alternatives) show varying degrees of success for Pythium root rot on many plants. This is not due to resistance but rather variable ability of the phosphonates to work on Pythium spp.
It is important to remember to watch other crops too. The change from summer to fall inevitably results in over-watered or drought-stressed plants. It is hard to water appropriately for weather that changes frequently and unpredictably. Plants that are pot-bound are especially sensitive since they have maximized their growth in their containers and really have little buffer to withstand water extremes. Now is the time to train new employees concerning the water needs of each crop.
Other Diseases of Potted Flowering Crops
Many of the other potted flowering crops are subject to their own unique set of diseases as summarized in the table above.
Conclusions
Disease prevention and control is never simple and relies on knowing your plants very well. Prevention always works better than eradication so know what to expect from each crop and apply preventative strategies wherever possible. And remember that prevention does not always mean applying a chemical fungicide. Consider cultural controls and biologicals first.
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