North Carolina Plant Disease and Insect Clinic
Plant Disease Fact Sheets

Pythium Root Rot in Tobacco Greenhouses

Tobacco greenhouse affected with Pythium root rot. The yellow patches are trays with severe symptoms of Pythium root rot.

Yellow foliage of tobacco seedlings is a symptom of severe Pythium root rot occurrence.

Brown roots is another symptom of Pythium root rot.

Structures of Pythium spp.

Pythium structures in a tobacco root (microscopic observation).

Mina Mila and W. A. Gutierrez


This disease has become very important for tobacco seedlings because the mechanism of Pythium dissemination is very adapted to float systems used in tobacco production in greenhouses.  Zoospores can swim and therefore 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 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 (Figure 1). The lower leaves turn yellow and plants wilt (Figure 2). Roots become light brown to gray colored in the early stages of infection (Figure 3). 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.

Causal Organism

Five species of Pythium have been identified as possible pathogens affecting tobacco transplants: Pythium myriotylum, P. dissotocum, P. irregulare, P. volutum, and P. spinosum. 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.

Pathogen Biology

The pathogen, Pythium, lives in water, where it will produce sporangia, the fruiting body (Figure 4). The sporangia will form zoospores which will swim around roots, infect, and colonize them. Zoospores are the main means of dissemination of this pathogen. Pythium will also form resting structures (chlamydospores and oospores) that will allow them to survive in pieces of infected roots trapped in the crevices of the styrofoam trays (Figures 5,6). 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 starts. 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.

Disease Management

Tray sanitation. Thoroughly wash previously used trays and 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. 
Water quality. Do not use water from ponds or creeks in your greenhouses.  Water from these sources could be primary sources of inoculum for Pythium
Fungicides.  See NC Agricultural manual:


Other resources:


Creation Date: 2001

Revision Date: April 2010

Key Words: tobacco, disease, greenhouse, root rot, Pythium

Glossary Terms: zoospores, chlamydospores, oospores, resting structures