First Report of Pythium myriotylum Causing Root and Stem Rot on Tobacco in Zimbabwe

Pythium species have a wide host range and are important pathogens of many agricultural crops. In Zimbabwe, 15 isolates of Pythium have been obtained from symptomatic tobacco (Nicotiana tabacum) in the new float seedling production system. This production system now accounts for 25 to 30% of the tobacco industry's annual requirement of 975 million seedlings. Disease symptoms are observed usually 5 to 6 weeks after sowing as wilting and yellowing of leaves followed by rotting of the roots, and in severe cases, seedling death. Up to 70% seedling loss has been reported in commercial seedbeds. In a study to fulfill Koch's postulates and to determine the susceptibility of 16 commercially grown tobacco cultivars, seedlings were produced in float trays and inoculated individually with 1 ml of Pythium (Isolate Py 19) spores (1 × 10⁴ CFU/ml) and mycelium pipetted around the base of the stem of each seedling 9 weeks after sowing. First symptoms appeared 7 to 10 days after inoculation as yellowing and wilting of leaves. When seedlings were pulled, the lower portion of the stem and roots were brown and rotted. Seedling mortality averaged 29% and disease incidence was 96 to 100% among cultivars. All 16 tobacco cultivars were susceptible to Pythium root and stem rot and there were no significant (P > 0.05) differences in their susceptibility to the disease. The pathogen was reisolated from the inoculated seedlings. The representative isolate (Py 19) sent to Centraalbureau voor Schimmelcultures, the Netherlands was identified as Pythium myriotylum Drechsler (CBS Accession No. 125021) (2). P. aphanidermatum, causing black stem rot, and P. debaryanum and P. ultimum, responsible for damping-off (3), have been reported in the predominant conventional soil-based tobacco seedling production system, but do not cause economic losses. However, stem and root rot caused by P. myriotylum threaten the float seedling production system in Zimbabwe, although a chemical curative control of the disease has been recommended and is now widely practiced. P. myriotylum has previously been reported in the tobacco float seedling production system in South Carolina (1). To our knowledge, this constitutes the first published report of P. myriotylum on tobacco in Zimbabwe. References: (1) M. G. Anderson et al. Plant Dis. 81:227, 1997. (2) Centraalbureau voor Schimmelcultures. Retrieved from http://www.cbs.knaw.nl , 2010. (3) A. J. Masuka et al. List of Plant Diseases in Zimbabwe. Department of Research and Extension and Tobacco Research Board. Harare, Zimbabwe, 2003.