Resistance in sunflower to 'Sclerotinia minor'

Abstract

'Sclerotinia sclerotiorum' and 'Sclerotinia minor' infect sunflower crops in Australia. The first fungus is mostly associated with head rots of sunflower crops growing in Queensland and northern New South Wales during cooler times of the year. 'S. minor', in contrast, is limited to root and basal stem rots leading to wilt in the irrigation areas of northern Victoria and southern New South Wales as well as small areas of the Liverpool Plains area of New South Wales. It can occur throughout the growing season. Damage caused by head rot can be reduced by ensuring that crops are not flowering during periods when the likelihood of conditions favourable for carpogenic germination of sclerotia is high. Control of sclerotinia wilt caused by 'S. minor' has not been adequately achieved in Australia. Plant breeding is one approach that might help to control of this disease. A review of the literature showed that many different methods have been used to screen sunflower for resistance to sclerotinia wilt caused by 'Sclerotinia sclerotiorum'. These methods could readily divide sunflower lines into groups on the basis of susceptibility. However, no single screening method has gained general acceptance as being the most efficient and effective means of selection for resistance to sclerotinia wilt. To increase the amount of useful data that could be collected for each inoculation, inoculum was placed in the bases of pots after careful removal of the root ball which was then replaced. The time taken from inoculation to the appearance of the a basal stem lesion, the rate of lesion extension on the stem and the proportion of the population killed were recorded. Incubation temperature, inoculum age and plant maturity were shown to affect the susceptibility of sunflower to 'S.minor' but quantity of inoculum used did not.