Identification and management of toxigenic fusarium species associated with fusarium head blight and fusarium crown rot of wheat in South Africa

Thesis (PhDAgric)--Stellenbosch University, 2018.

ENGLISH ABSTRACT: Fusarium head blight (FHB) and Fusarium crown rot (FCR) are important diseases of wheat. FHB, which is caused by members of the Fusarium graminearum species complex (FGSC), reduces yields and grain quality, and contaminates grain with mycotoxins (toxic secondary fungal metabolites) like deoxynivalenol (DON), nivalenol, and zearalenone (ZEA). FCR, caused by Fusarium pseudograminearum, infects the lower stem and causes stem rotting, which also results in yield losses. Fusarium pseudograminearum can also cause FHB. Management of FHB involves integrating agronomical practices (crop rotation, tillage), host resistance and chemical control. In the irrigation regions of South Africa, however, producers rotate wheat with summer crops like maize, which also hosts the FGSC, while crop rotation and minimum/no-till is common in the Western Cape. These practices necessitates the use of host resistance and chemical control to manage FHB in South Africa. Accurate identification of pathogens causing FHB and FCR in South Africa is needed to determine their distribution and to assist breeders in developing resistant wheat cultivars. Comprehensive surveys were thus conducted over 2 years to identify Fusarium species and their chemotypes in the country. The FGSC with the 15-cetyldeoxynivalenol chemotype predominated in the irrigation regions of South Africa, while F. pseudograminearum was the dominant species in the Western Cape region. Commercial irrigation and dryland wheat cultivars, as well as test lines with different quantitative trait loci (QTL), were evaluated for resistance to FHB by field-inoculation with F. graminearum s.s. and F. pseudograminearum. Among the irrigation cultivars, Krokodil, SST843, SST866, and SST884 performed the best. The Fhb1 QTL had a marked effect on the percentage Fusarium-damaged kernels, and reduced DON content in most test lines. One test line, BFUS2011-17, with no QTLs from Sumai 3, grouped among the top performers. Disease levels were low in dryland trials, but DON and ZEA levels were high in the first year, with the resistant control and SST027, SST056 and SST087 containing low mycotoxin levels. The efficacy of three commercial fungicides (Abacus, Amistar Xtra and Prosaro) and two chemical seed treatments (Galmano Plus and Vitavax Plus) to manage FHB and increase grain quality was determined in field trials over 2 years. All foliar treatments reduced disease and DON levels, while significantly improving grain quality and yield. Seed treatments had little effect on FHB, but Abacus combined with seed treatments reduced disease incidence more than Abacus alone. Prosaro combined with Galmano Plus reduced yields compared to Prosaro alone. This study provided the first comprehensive report of Fusarium species and their type B trichothecene chemotype associated with FHB and FCR in South Africa. Irrigation cultivars, test lines and dryland cultivars with improved resistance to FHB and DON contamination were identified, while fungicides reduced FHB and DON, and improved grain quality. Future studies should determine the natural occurrence of mycotoxins in wheat fields throughout South Africa, develop resistant irrigation and dryland cultivars, determine the efficacy of chemical control of FHB in different environments over multiple years, and optimise chemical control of FHB through timing of application, different nozzles and adjuvants.

AFRIKAANSE OPSOMMING: Fusarium aarskroei (aarskroei) en Fusarium kroonvrot (kroonvrot) is belangrike koringsiektes. Aarskroei word deur lede van die Fusarium graminearum spesiekompleks (FGSK) veroorsaak, en lei tot verlaagde opbrengste en graankwaliteit, en besmetting van graan met mikotoksiene (toksiese sekondêre swam-metaboliete) soos deoksinivalenol (DON), nivalenol, en zearalenone (ZEA). Kroonvrot word deur Fusarium pseudograminearum veroorsaak, wat die laer stam infekteer en stamverrotting veroorsaak, en ook oesverliese tot gevolg het. Fusarium pseudograminearum kan ook aarskroei veroorsaak. Bestuur van aarskroei behels die integrasie van agronomiese praktyke (wisselbou, bewerking), gasheerweerstand, en chemiese beheer. Produsente in die besproeiingstreke van Suid-Afrika wissel koring egter met somergewasse soos mielies af, wat ook die FGSK kan huisves, terwyl wisselbou en minimum / geenbewerking algemeen in die Wes-Kaap is. Hierdie praktyke noodsaak die gebruik van gasheerweerstand en chemiese beheer om aarskroei in Suid-Afrika te bestuur. Akkurate identifisering van patogene wat aarskroei en kroonvrot in Suid-Afrika veroorsaak is nodig om hul verspreiding te bepaal, en om telers by te staan in die ontwikkeling van weerstandbiedende kultivars. Omvattende opnames is gevolglik oor 2 jare uitgevoer om Fusarium spesies en hul chemotipe in Suid-Afrika te bepaal. Die FGSK met die 15-asetieldeoksinivalenol chemotipe was dominant in die besproeiingstreke van Suid-Afrika, terwyl F. pseudograminearum in die Wes-Kaap gedomineer het. Kommersiële besproeiings- en droëland-kultivars, sowel as toetslyne met verskillende komplimente kwantitatiewe lokuskenmerke (QTL), was vir weerstand teen aarskroei d.m.v. veld- inokulasies met F. graminearum s.s. en F. pseudograminearum ge-evalueer. Van die besproeiings-kultivars het Krokodil, SST843, SST866 en SST884 die beste gevaar. Die Fhb1 QTL het ‘n merkbare effek op die persentasie Fusarium-beskadigde korrels gehad, en het die DON-inhoud in meeste toetslyne verlaag. Een toetslyn, nl. BFUS2011-17, wat geen QTL van Sumai 3 het nie, was onder die top-resteerders. Siekte-vlakke in die droëland-proewe was laag, maar DON en ZEA vlakke was baie hoog in die eerste jaar, terwyl die weesstandbiedende kontrole, SST027, SST056 en SST087 lae mikotoksien-vlakke gehad het. Die effektiwiteit van drie kommersiële fungisiedes (Abacus, Amistar Xtra en Prosaro) en twee chemiese saadbehandelings (Galmano Plus en Vitavax Plus) om aarskroei te bestuur en graankwaliteit te verbeter, was m.b.v. veldproewe oor twee jare bepaal. Alle blaarbehandelings het siekte- en DON-vlakke verlaag en graankwaliteit betekenisvol verbeter. Saadbehandelings het weining effek op aarskroei gehad, alhoewel Abacus tesame met saadbehandelings die siekte- insidensie meer verlaag het as Abacus alleen. Prosaro tesame met Galmano Plus het die opbrengs verlaag vergeleke met Prosaro alleen. Hierdie studie was die eerste omvattende verslag oor Fusarium spesies en hul tipe B trichothecene chemotipe geassosieer met aarskroei in Suid-Afrika. Besproeiings-kultivars, toetslyne en droëland-kultivars met verbeterde weerstand teen aarskroei en DON besmetting is geidentifiseer, terwyl fungisiedes aarskroei en DON verminder en graankwaliteit verbeter het. Toekomstige studies moet die natuurlike vlakke van mikotoksiene in koringlande regoor Suid-Afrika bepaal, weerstandbiedende besproeiings- en droëland-kultivars ontwikkel, die effektiwiteit van chemiese beheer van aarskroei in verskillende omgewings oor verskeie jare bepaal, en chemiese beheer van aarskroei optimiseer d.m.v. tydsberekening van toediening, sowel as verskillende spuitpunte en benatters.

Doctoral