Fusarium dry rot of potatoes: etiology, epidemiology, toxicity and control

English: Eighteen Fusarium species were isolated from dry- and stem-end-rotted potato tubers in South Africa. Samples with dry-rot lesions were easily obtained, even though samples were taken directly after harvest and Fusarium dry rot is a post-harvest disease, highlighting the importance of this disease in South Africa. Nine species (F. oxysporum, F. so/ani, F. sambucinum, F. acuminatum, F. crookwellense, F. graminearum. F. culmorum, F. scirpi and F. equiseti) caused typical dry-rot lesions on artificially inoculated potato tubers. Fusarium oxysporum and F. solani were predominantly isolated and the most virulent. These results differ from those obtained in other parts of the world, especially the northern hemisphere, where F. sambucinum and F. solani are mainly associated with Fusarium dry rot. Control of Fusarium dry rot with mancozeb or thiabendazole, the only fungicides registered for control of this disease in South Africa, was unsatisfactory. This was especially evident when F. oxysporum was the cause of the rotting. It is recommended that effective fungicides against F. solani and F. oxysporum, e.g. prochloraz, should be applied as soon as possible after harvest and that potatoes should be stored at 5°C. Thiabendazole resistance, which is a problem in North America and Europe, appears to be absent in local isolates of F. solani, F. oxysporum and F sambucinum. The use of seed pieces under local conditions is risky. When used due to scarcity of seed tubers or economic reasons, good quality seed tubers should be used and treated with carbendazim on the day prior to a spring planting under irrigation in soils with a temperature <18°C. Fusarium solani [= F. solani var. coeruleum and F. coeruleumi is commonly associated with dry rot of potatoes world wide. The local F. solani isolates were compared to foreign F. solani, F. solani var. coeruleum and F. coeruleum isolates. Local and foreign F solani isolates did not differ morphologically and with respect to their optimum growth temperatures and matched the description of F. solani. However, foreign isolates referred to as F. solani var. coeruleum and F. coeruleum differed from the F. solani isolates. Grouping these isolates together under one species (F. solani) is thus disputable. In contrast to other countries, Fusarium sambucinum [= F. sulphureum and F. roseum var. sambucinum J, is not commonly associated with dry rot of potatoes in South Africa. The local isolates were compared with foreign isolates because they appeared to be atypical of F. sambucinum sensu lata. Moreover F. sambucinum sensu lato has been divided into three species i.e. F. sambucinum sensu srticto, F. torulosum and F. venenatum. Morphologically and on the basis of optimum temperature for growth and dry-rot development, the local isolates could not be differentiated from the foreign isolates, but no vegetative compatibility occurred between them. Local and foreign isolates could be assigned to six and three vegetative compatibility groups, respectively. Sexual crosses between the local isolates and the Gibberella pulicaris strains, were unsuccessful. However, crossing some local isolates with each other resulted in fertile crosses, producing ascospores significantly larger than those produced when the G. pulicaris strains were crossed. This is the first report of South African F. sambucinum isolates from potatoes producing a teleomorph. Five of the local F. sambucinum isolates generated bands of the expected size with F. venenatum primers, indicating that these isolates are conspecific with F. venenatum. However, morphologically these isolates did not agree with the description of F. venenatum and their growth rates were faster. It appears that these South African isolates are members of a new species of Fusarium with a newly discovered Gibberella. The genus Fusarium contains important mycotoxin-producing species which have been implicated in human and animal diseases. Diseased or damaged tubers are often fed to cattle, stressing the urgency to determine the possible threat posed to the industry. Except for F. graminearum Gr. 1, isolates of the other 17 Fusarium species tested, caused death in one-day-old Pekin ducklings, indicating the involvement of mycotoxins. Differences in the toxicity of the various Fusarium spp., as well as within isolates of the same species, were evident. Fusarium nygamai appeared to be the most toxic, followed by F. acuminatum and F. moniliforme. The fact that South African dry-rot isolates proved to be toxic to ducklings emphasizes the need to determine the identity of toxins produced, particularly if these mycotoxins can be produced in colonized potato tubers. This dissertation has made a contribution towards the better understanding of, and the development of an integrated control strategy for Fusarium dry rot of potatoes in South Africa.