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

[摘要] English: Eighteen Fusarium species were isolated from dry- and stem-end-rotted potato tubers inSouth Africa. Samples with dry-rot lesions were easily obtained, even though samples were takendirectly after harvest and Fusarium dry rot is a post-harvest disease, highlighting the importanceof 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) causedtypical dry-rot lesions on artificially inoculated potato tubers. Fusarium oxysporum and F. solaniwere predominantly isolated and the most virulent. These results differ from those obtained inother parts of the world, especially the northern hemisphere, where F. sambucinum and F. solaniare mainly associated with Fusarium dry rot.Control of Fusarium dry rot with mancozeb or thiabendazole, the only fungicidesregistered for control of this disease in South Africa, was unsatisfactory. This was especiallyevident when F. oxysporum was the cause of the rotting. It is recommended that effectivefungicides against F. solani and F. oxysporum, e.g. prochloraz, should be applied as soon aspossible after harvest and that potatoes should be stored at 5°C. Thiabendazole resistance, whichis 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 useddue to scarcity of seed tubers or economic reasons, good quality seed tubers should be used andtreated with carbendazim on the day prior to a spring planting under irrigation in soils with atemperature <18°C. Fusarium solani [= F. solani var. coeruleum and F. coeruleumi iscommonly associated with dry rot of potatoes world wide. The local F. solani isolates werecompared to foreign F. solani, F. solani var. coeruleum and F. coeruleum isolates. Local andforeign F solani isolates did not differ morphologically and with respect to their optimum growthtemperatures and matched the description of F. solani. However, foreign isolates referred to asF. solani var. coeruleum and F. coeruleum differed from the F. solani isolates. Grouping theseisolates together under one species (F. solani) is thus disputable.In contrast to other countries, Fusarium sambucinum [= F. sulphureum and F. roseumvar. sambucinum J, is not commonly associated with dry rot of potatoes in South Africa. The localisolates 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 thebasis of optimum temperature for growth and dry-rot development, the local isolates could notbe 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, wereunsuccessful. However, crossing some local isolates with each other resulted in fertile crosses,producing ascospores significantly larger than those produced when the G. pulicaris strains werecrossed. This is the first report of South African F. sambucinum isolates from potatoes producinga teleomorph. Five of the local F. sambucinum isolates generated bands of the expected size withF. 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 theirgrowth rates were faster. It appears that these South African isolates are members of a newspecies of Fusarium with a newly discovered Gibberella.The genus Fusarium contains important mycotoxin-producing species which have beenimplicated 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-oldPekin ducklings, indicating the involvement of mycotoxins. Differences in the toxicity of thevarious Fusarium spp., as well as within isolates of the same species, were evident. Fusariumnygamai appeared to be the most toxic, followed by F. acuminatum and F. moniliforme. The factthat South African dry-rot isolates proved to be toxic to ducklings emphasizes the need todetermine the identity of toxins produced, particularly if these mycotoxins can be produced incolonized potato tubers.This dissertation has made a contribution towards the better understanding of, and thedevelopment of an integrated control strategy for Fusarium dry rot of potatoes in South Africa.

[发布机构] University of the Free State