FACTS ABOUT PYTHIUM & PHYTOPHTHORA In greenhouses and nurseries, pathogens proliferate in plant debris and can be introduced into growing media by soiled hands, tools, flats and colonized transplants.
Signs of Pythium: Pythium is commonly associated with excessive nutrient levels or ammonium toxicity. With a genus comprised of more than 100 species, pythium can lead to any number of diseases, among them crown and stem rot and damping-off.
Damping-off affects seeds and new seedlings by rotting stem and root tissues at and below the soil surface. Though infected plants may germinate problem-free, within a few days they can become mushy and water-logged. Species of phytophthora can cause damping-off, but poor soil drainage and high humidity are more likely to invite pythium into the grow space.
“Pythium is not a true fungi, it’s a water mold,” says Alicyn Smart, a plant pathologist and director at the University of Maine’s plant disease diagnosis laboratory. “With damping-off, another symptom for growers to look out for is when the plant flops over at the seedling stage.”
Non-uniform crop height is also a telltale sign of pythium, particularly for groups of crops planted around the same time. Soil with high soluble salt content is vulnerable to the pathogen, Hausbeck notes. Being a water mold, pythium will spread more rapidly when soil is saturated, or if staff members don’t adjust watering routines after a series of overcast days.
Potting Phytophthora: Although soaked soil and high nitrogen content provide optimum conditions for phytophthora to thrive, unattended plant material is thought to be the pathogen’s most likely source of origin. Phytophthora is not usually found in commercial seed, nor does it travel easily through air over long distances.
Fungicides: Rotating fungicides, which requires alternating products with different modes of action, is crucial in limiting the amount of time a pathogen is exposed to any single product, says Beckerman. For best results, growers should also avoid consecutive applications of fungicides within the same chemical class.
“When you do use fungicides, make sure to apply the recommended dose as listed,” says Janna Beckerman, a professor of botany and plant pathology at Purdue University. “Skipping a spray is the worst thing you can do, because it allows the population to come back up.”
Insect vectors: University of Maine’s Alicyn Smart, a plant pathologist and director at the school’s plant disease diagnosis laboratory, points to fungus gnats and shore flies as another vector for root-damaging disease. These annoying insects are attracted to damp locations where fungi are apt to flourish, Smart says. While adult fungus gnats are more of a nuisance, their larvae feed on plant roots and decaying organic matter. Feeding damage creates wounds that allow soilborne pathogens, including pythium, to enter and kill plants. The larvae themselves may also carry deadly pathogens, making swift eradication important for growers.
Cultural controls: Generally, owners should check their roots regularly, either during watering or when examining foliage.
“Growers should actually take plants out of containers to look at roots,” Smart says. “They should also know what healthy roots look like. Good ones will be white and fleshy, while diseased roots may be brown or degrade in your hands. If you have any previous issues with root disease, you should look at them more often.”
Beckerman says too many growers choose to simply dispose of a few infected plants rather than taking a more merciless approach.
“That will just allow the pathogen to persist and require additional fungicide applications,” says Beckerman. “If you have to cull, be ruthless.”
Once the disease is destroyed, an increased focus on cultural controls can help stave off a potentially disastrous return, including inspecting incoming plant material, avoiding overwatering and controlling insect pests and weeds.
Sanitation best practices can be as simple as keeping a watering hose off any surface that collects soil. Smart suggests cleaning reusable trays with one part bleach and nine parts water, as well as disinfecting tools. Disinfect benches and walkways. Steam-cleaning trays and pots is another option.
“After removing the disease, sanitation and good cultural practices will alleviate reliance on fungicides,” says Smart.
This article originally appeared in sister publication Greenhouse Management
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