Marasas, W. F. OWingfield, M. J.Theron, Daniël Jacobus2017-05-222017-05-221999-05http://hdl.handle.net/11660/6265English: 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.Afrikaans: Agtien Fusarium spesies is uit aartappels met droëvrot en puntjievrot simptome in Suid- Afrika geïsoleer. Alhoewel droëvrot 'n na-oes siekte is, is aartappels met droëvrotsimptome geredelik direk na oes, gevind. Dit bevestig die belangrikheid van Fusarium droëvrot in Suid- Afrika. Nege spesies (F. oxysporum, F. solani, F. sambucinum, F. acuminatum, F. crookwellense, F. graminearum, F. culmorum, F. scirpi en F. equiseti) het tipiese droëvrot simptome in kunsmatig geïnokuleerde aartappelknolle veroorsaak. Fusarium oxysporum en F. solani was die primêre asook die mees virulente spesies geïsoleer. Die resultate verskil van wat elders in die wêreld gevind word, veral lande in die noordelike halfrond, waar F. sambucinum en F. solani hoofsaaklik met Fusarium droëvrot geassosieer word. Beheer van Fusarium droëvrot met mankoseb en tiabendasool, die enigste fungisiedes vir die beheer van die siekte in Suid-Afiika geregistreer, was teleurstellend, veral waar F. oxysporum die verrotting veroorsaak het. Fungisiedes wat tenminste teen beide F. oxysporum en F. solani effektiefis, bv. prochloras, moet so gou moontlik na oes toegedien word waarna die aartappels by 5 °C opgeberg moet word. Bestandheid teen tiabendasool, wat 'n probleem in Noord-Amerika en Europa is, kon nie in plaaslike F. oxysporum, F. solani en F. sambucinum isolate gevind word nie. Die plant van gesnyde moere onder plaaslike toestande word nie aanbeveel nie. Indien gesnyde moere weens ekonomiese redes wel geplant word, moet slegs goeie kwaliteit moere, wat die dag voor plant gesny en met karbendasim behandel is, tydens lenteplantings in gronde onder besproeiing met temperature < 18 0 C geplant word. Fusarium solani [= F. solani var. coeruleum en F. coeruleum] word wêreldwyd met droëvrot van aartappels geassosieer. Plaaslike F. solani isolate is met buitelandse F. solani, F. solani var. coeruleum en F. coeruleum isolate vergelyk. Plaaslike en buitelandse F. solani isolate verskil nie morfologies en ten opsigte van huloptimum temperature vir groei van mekaar nie en stem met die beskrywing van F. solani ooreen. Die F. solani var. coeruleum en F. coeruleum isolate verskil egter van die F. so/ani isolate. Om die isolate dus onder een spesie (F. so/ani) te groepeer word bevraagteken. In teenstelling met ander lande word Fusarium sambucinum [= F. sulphureum en F. roseum var. sambucinum] nie algemeen met droëvrot van aartappels in Suid-Afrika geassosieer nie. Die plaaslike isolate is met die buitelandse isolate vergelyk omdat hulle oënskynlik atipies van die F. sambucinum sensu lata is, en omdat 19. tans in F. sambucinum sensu stricto, F. torulosum en F. venenatum opgedeel word. Morfologies en op grond van huloptimum temperature vir groei en droëvrot ontwikkeling, kon die plaaslike en die buitelandse isolate nie van mekaar onderskei word nie. Vegetatiewe verenigbaarheid was egter afwesig tussen die twee groepe isolate. Die plaaslike en buitelanse isolate kon in onderskeidelik ses en drie vegetatiewe verenigbaarheids groepe verdeel word. Geslagtelike kruisings tussen die plaaslike- en buitelandse Gibberella pulicaris isolate was ook onsuksesvol. Sommige plaaslike isolate kon egter suksesvol met mekaar gekruis word en dié askusspore geproduseer was betekenisvol groter as dié geproduseer tydens kruisings tussen die buitelandse Gibberella pulicaris isolate. Dit is die eerste geval waar Suid- Afrikaanse F. sambucinum isolate, vanaf aartappels, 'n teleomorf produseer. Vyf plaaslike F. sambucinum isolate het bande van verwagte grootte met F. venenatum voorvoerders geproduseer wat impliseer dat die isolate met die van F. venenatum ooreenstem. Morfologies stem die isolate egter nie met die beskrywing van F. venenatum ooreen nie en was hul groeispoed ook vinniger. Dit blyk dus dat die Suid-Afrikaanse isolate deel van 'n nuwe spesie van Fusarium is, waarvan 'n nuwe Gibberella ontdek is. , I '1 Il Belangrike rnikotoksien produserende spesies kom in die genus Fusarium voor wat met mens- en diersiektes geassosieer word. Die feit dat beskadigde of besmette aartappelknolle algemeen vir vee gevoer word, beklemtoon die noodsaaklikheid om die gevaar wat dit vir die bedryfinhou, te bepaal. Met die uitsondering van dieF. graminearum Gr. I isolate, was die ander 17 Fusarium spesies wat getoets is, in staat om dagoud Pekin eendjies te dood. Dit is 'n aanduiding dat mikotoksiene betrokke is. Toksisiteit verskille het tussen die Fusarium spesies, sowel as tussen isolate van die selfde spesie voorgekom. Fusarium nygamai blyk om die mees toksiese spesie te wees, gevolg deur F. acuminatum en F. moniliforme. Die feit dat Suid- Afrikaanse droëvrot-isolate toksies vir eendjies is, beklemtoon die noodsaaklikheid om die identiteit van die toksiene geproduseer te bepaal, veral sou die rnikotoksiene in gekoloniseerde aartappels geproduseer word. Die tesis het tot 'n beter begrip van, en die ontwikkeling van 'n itegreerde beheerprogram teen Fusarium-droëvrot van aartappels in Suid-Afrika bygedra.enFusarium -- South AfricaFungal diseases of plants -- South AfricaPotato -- Diseases and pests -- South AfricaThesis (Ph.D. (Plant Pathology))--University of the Free State, 1999ThesisUniversity of the Free State