Chemical and biological control of Pythium root rot of tobacco seedlings

There has been a substantial increase in the utilization of float systems to produce tobacco (Nicotiana tabacum) seedlings for transplant by tobacco producers within the last ten years. In Tennessee alone, 80% of transplants were produced in the float system in 1999. Pythium species are some of the most common and destructive root-infecting pathogens of tobacco grown in float culture. They cause damping-off and root rot of seedlings. Pythium spp. infect seedlings by motile spores called zoospores. Characteristic symptoms of Pythium disease are stunting, yellowing and wilting of leaves and severe reduction of the root system. Besides cultural practices, such as sanitation in and around float beds, control options are limited. There are no cultivars that are resistant to Pythium, and until recently, there were no fungicides registered for control of Pythium diseases in the tobacco float bed system. The objectives of this research were to investigate various rates, methods of application, and time of application of the fungicide etridiazole, a rhamnolipid biosurfactant produced by Pseudomonas aeruginosa, and chitosan, a non-toxic oligomer of β-1,4- glucosamine purified from shrimp shells, on Pythium disease of tobacco caused by P. myriotylum. The specific aims were to determine the effect of these products on (i) seed germination, (ii) disease severity, (iii) levels of phytotoxicity to burley tobacco (cv. TN90) seedlings; and (iv) the viability of P. myriotylum after the treatment. Etridiazole applied in float water at 0.075 and 0.150 g l^-1 on the same day Pythium was inoculated provided the most effective control of Pythium root rot without compromising plant growth or germination rate. Etridiazole at a rate of 0.56 g l^-1 applied as soil drench on the potting soil surface on the same day Pythium was inoculated also provided control, and shoot growth was increased, but root growth was less than etridiazole treatments applied in float water. Incorporation of etridiazole at 0.11, 0.22 and 0.33 g l^-1 into growth medium resulted in the lowest seed germination, and phytotoxicity of the seedlings was noted in some replicates in one of two trials. Application of a rhamnolipid biosurfactant at 8 and 12 µ l^-1 by soil drench, in float water, and by tray immersion at 2, or 3 wk after seeding, and at inoculaton of Pythium did not provide effective control of Pythium root rot. Chitosan was applied in float water as a solution 24 h before the inoculation of Pythium. Chitosan applied at 400 and 500 µg ml^-1 suppressed Pythium root rot in this study. The 500 µg ml^-1 treatment resulted in a lower disease rating than the 400 µg ml^-1 treatment, however, there were no differences in mean shoot and root weight. Disease rating and plant growth from chitosan at 500 µg ml^-1 were comparable to treatment with 0.15 g l^-1 etridiazole applied to float water. Unlike the fungicide etridiazole, the mechanism of action of chitosan is primarily induction of plant natural resistance mechanisms.