Pythium root rot damages transplants

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By Rick Greenwell

Besides blank shank, this is the worst disease we have to deal with.  In the float systems, occasionally this is fatal, but mostly what it does is rob you of time.  With the serious outbreak of this disease it will set the affected plants back four to six weeks.  Most people can’t afford that kind of waiting period.  So, please pay attention to this article and manage accordingly.

Pythium root rot (PRR) is the most common damaging disease that we encounter in the float system, and it won’t be long before tobacco transplants around Kentucky are affected by this problem.

The first symptoms of Pythium root rot tend to be yellowing and stunting of transplants in a well defined area or areas of a float bay. Damping-off, or seeding death, can occur in severe cases.  During the outbreak, seedlings wilt and root systems decay to some degree. Roots and sometimes lower stems of plants affected by Pythium root rot take on a darkened, necrotic appearance; roots may have a slimy appearance.  Infected roots will eventually slough off and some re-growth may be observed; however, new growth likely will become infected.

Water temperatures greater than 72°F favor rapid development and spread of PRR in float systems.  Several species Pythium, a fungus-like organism, are known to cause root rots on tobacco seedlings. Pythium species (spp.) require water, abundant in the float system, for reproduction and movement initial infections likely result from germination of resting structures (oospores) of Pythium (spp)., and production of zoosporangia.  Swimming spores (zoospores) are liberated from zoospores encyst after encountering susceptible tissue and enter the root system to establish an infection.  Many cycles of zoospore production and infection are possible after initial infections occur.

The most common ways for Pythium spp. to be introduced into float systems are contaminated water, infested soil, or recycled (and contaminated) water Styrofoam trays.  Pythium spp. Are found widely in our soils and surface water and can be carried on shoes or implements.  Pythium spp. can persist in the tissue of roots that have penetrated Styrofoam float trays, providing a source of inoculums when the trays are used the following season.

Sanitation is an important part in the management of Pythium root rot in the float system.  Never use pond or surface water to fill float beds, since water from these sources is likely to contaminate with Pythium and other plant pathogens such as Phytophthora or Fusarium.  Make sure that shoes and tools are cleaned before bringing them into a transplant facility.

Terramaster $EC is labeled for use in float systems and is very effective against PRR when used correctly.  Detailed information on this fungicide can be found in the product label.  We have publications at the office to help advise you.  For preventive use, apply 0.7 – 1 fl oz of product per 100 gallons of float water beginning 2-3 weeks after seeding, or when roots first enter the water.  A second treatment of 0.7 – 1 fl oz of product per 100 gallons for float water  2-3 weeks after seeding, or when roots first enter the water.  A second treatment of 0.7-1 fl oz per 100 gallons of water can be made 3 weeks after the firs, and a final application of 0.8 fl oz can be made after seeding; make sure that the product is mixed thoroughly in float bays to minimize the risk of plant injury.  “Rescue” applications of Terramaster (1.4 fl oz/100 gallons of float water) in systems with active PRR will halt the further development of disease and symptomatic seedlings will likely recover.  However, the higher rates of Terramaster used in rescue treatments increase the risk of plant injury AND recuperating plants may still harbor Pythium and increase their susceptibility to black shank and Fusarium wilt.  For these reasons, preventive use of Terramaster is recommended over curative applications of the product.  Before using Terramaster, or any pesticide, refer to the label for specific instructions and safety information.  Quality tobacco transplants are one of the most important parts of a successful growing season.  Through careful management it is possible to achieve excellent control of PRR, good transplant quality, and a healthy bottom line.