Here in my first Kentucky springtime, Pythium diseases top my list of frequently-sighted problems in both vegetable and tobacco systems. Pythium can cause pre- and post-emergent damping-off and root rots on larger plants.
What to Look For
With pre-emergent damping-off, planted seeds will rot in the media and plants may never be seen. With post-emergent damping-off, seeds germinate, seed leaves expand, and sometimes a few true leaves develop, but the plant soon succumbs to the pathogen. If a suspected Pythium-infected stem is inspected closely, you may sometimes observe discoloration and stem girdling at the soil line (Figures 1 and 2). Inspection of the root system will show soft, darkened roots, and by gently pulling a root between two fingers, the outer layers will often slough off from the center (also known as the stele).
Pythium is not a true fungus; it is fungus-like organism in the oomycete (water molds) group. Wet weather experienced earlier this spring favored pathogen populations in many of our growing systems. Pythium has a broad host range and can cause disease in a wide variety of plants, at any stage of development. Pythium-infected transplants that do not damp-off oftentimes will not thrive in the field due to pathogen-induced root rots. Plants with Pythium root rot may have symptoms of nutrient deficiency (due to poorly developed root systems) or they may be stunted compared to neighboring plants (Figure 3). This will be especially apparent in low-lying areas of the field or high tunnel where conditions are most favorable. Infected plants will not fruit as heavily as uninfected plants, or the fruit may be smaller or of poorer quality.
Pythium root rots are very common in the tobacco float bed system (Figure 4), and a number of infected vegetable transplant samples have been identified this year as well.
To Manage Pythium In Tobacco Float Bed Systems:
Pythium may be managed by using either preventative (1.0 fl oz / 100 gal) or “curative” (1.4 fl. oz / 100 gal) rates of Terramaster applications stirred well into the float bed water. Many report better results when using Terramaster preventatively, but this product should only be used when tobacco roots enter the water, since earlier applications may set very young seedlings back.
To Manage Pythium In Greenhouse or High Tunnel:
With a 24(c) label, Terramaster may also be used to manage Pythium and Phytophthora root rots in greenhouse or high tunnel tomatoes, at a rate of 6.5 fl. oz / 500 gal. Another chemical option for Pythium management in either the high tunnel or the field is Previcur Flex, which may be applied through the drip, as a soil drench, or by directed spray at the base of plants. This product is labeled for use on tomatoes, peppers, cucurbits, and lettuce, providing a versatile option for Pythium management. Please be sure to check the Previcur Flex label for specific rates and other use instructions by crop.
Don’t Forget Cultural Practices
Steam disinfestation of planting mix is recommended to kill Pythium that may be present in the media. Open irrigation sources, such as surface pond water or creek water, should be avoided, since Pythium (and Phytophthora) pathogens can readily be baited out of these sources. Good water management, including avoiding overwatering, planting into raised beds, and preventing standing water in and between rows, will also help reduce the effects of Pythium in growing systems. The most effective control of Pythium diseases results from integrating chemical and cultural management tools throughout the growing season.
By Emily Pfeufer, Extension Plant Pathologist
Acknowledgement: Thanks to Julie Beale and Brenda Kennedy for their thoughtful editorial comments.