Pythium Root Rot in Tobacco Greenhouses
TB08 - Tobacco Disease Note No. 8
W. A. Gutierrez and T. A. Melton 

Most flue-cured tobacco growers produce transplants using the float tray system in greenhouses. In fact, production in North Carolina increased from 55% in 1994 to 98% in 1999.  Under these conditions, Pythium root rot has became one of the most important limiting factors in the production of seedlings.  This diseases has became very important for tobacco seedlings because the mechanism of Pythium dissemination is very adapted to float systems used in tobacco production in greenhouses.  The swimming zoospores can easily move throughout the seedling bed in the water and reach epidemic levels in a short time.  If infected plants live, their vigor is reduced, resulting in poor quality transplants. 

Symptoms of Pythium Root Rot

Symptoms are usually observed when seedlings are 25 days or older, when roots start to grow down to the bottom of the tray into the water.  First symptoms on seedling beds are observed as round, yellow areas that in a few days will cover an entire section of the greenhouse.  The lower leaves turn yellow and plants wilt. Roots become light brown to gray colored in the early stages of infection.  In a few days roots will become brown to gray with a slimy texture. These infected roots later will fall off, leaving seedlings without a root system. In some cases, white roots will regrow, only to be killed later.

The pathogen, Pythium, is very adapted to live in water, where it will produce sporangia, the fruiting body.  The sporangia will form zoospores (spores that are motile and are able to swim in the water) which will swim around roots, infect, and colonize them.  Zoospores are the main means of dissemination of this pathogen in this system.  Pythium will also form resting structures (chlamydospores and oospores) that will allow them to survive for long periods in soil, but under the float tray system in greenhouses it will survive in pieces of infected roots trapped in the crevices of the styrofoam trays.

Five species of Pythium have been identified as possible pathogens affecting tobacco transplants.  Pythium myriotylum, P. dissotocum, P. irregulare, P. volutum, and P. spinosum in greenhouses. P. myriotyilum and P. volutum are the most aggressive species affecting tobacco seedlings at water temperatures above 72 F.  No above ground symptoms have been observed with the other Pythium species at 72 F, but seedlings do have root rot and a poor root system at transplanting time.  Aggressive isolates can kill seedlings in a few days.  The severity of Pythium on tobacco seedlings will depend on the growth stage when the disease started.  Without controlling the disease, plants that get infected as late as 45 days after seeding and will not reach the adequate size and vigor needed for transplanting.

Control Recommendations

Tray sanitation. Thoroughly wash previously used traysand allow them to dry. Fumigate with methyl bromide at 3 lb/1000 cubic feet. Do not depend on dipping trays in any sanitation product, including bleach to kill fungal pathogens.  Considerations for using methyl bromide: Crisscross trays up to a 5 feet high, tarp and sealed on concrete or on a tarp, then fumigate.  Check that the air temperature is above 65 F at the time of the fumigation.  Do not fumigate inside the greenhouse.  Allow at least 48 hours of aeration before using trays.
Water quality. Do not use water from pond or creeks in your greenhouses.  Water from these sources could be primary sources of inoculum for Pythium. 
Fungicides.  Apply Terramaster 35 WP at 2 oz per 100 gallons of float water.  Be sure to premix the Terrazole and throughly distributed through the float water.

Additional pictures
Symptoms in greenhouses

• Initial infection and dissemination on seedling beds (Figure 1, 2)

• Symptoms in the foliage (Figure 3, 4)
• Symptoms on roots (Figure 5, 6)
• Symptoms on plants (Figure 7, 8)

• Sporangia, zoospores, oospores (Figure 9, 10, 11, 12)

For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service

North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Reformatted October 2001 by Plant Disease and Insect Clinic