Claviceps africana (ergot)

Cultural control is not a reliable control technique, often depending on the capricious nature of the climate. In Zimbabwe, crops may escape ergot if early rains permit sowing in November so that flowering both avoids pollen sterility induced by cool nights and coincides with a mid-season dry spell in January or February (Frederickson and Leuschner, 1997). Sorghum is unaffected by ergot when seed multiplication is performed under irrigation in the dry season at Mazarabani. Similarly, early sowings of sorghum avoid ergot infection in India (Singh, 1964; Sangitrao et al., 1979; Anahosur and Patil, 1982) and central Mexico (Montes-Belmont et al., 2002b).

Field practices aimed at reducing the risk or severity of infection include the removal of infected panicles at harvest, 3-year crop rotations and deep ploughing of field residues. However, despite these measures, a serious epiphytotic occurs every 5-10 years in Zimbabwe (Frederickson and Leuschner, 1997). Increasing the ratio of pollen-producing rows to the male-sterile, female parent, or staggering the planting dates of the pollen donor rows helped reduce ergot by increasing the period when pollen was available (Frederickson and Leuschner, 1997), but only if the weather conditions were favourable for pollination. Cold nights 2-3 weeks before flowering and cool, wet weather at flowering and during the 5 days after flowering (McLaren and Wehner, 1990; 1992) have an overriding negative effect on all planting systems, promoting disease.

There is currently no source of resistance to sorghum ergot for use in the field in A-lines. Resistant fertile sorghums have been reported (Tegegne et al., 1994; Musabyimana et al., 1995), but resistance has proved to be a function of cleistogamy, or fast and efficient pollination and fertilization (Bandyopadhyay, 1992; Frederickson et al., 1994) with no potential use in A-lines. In trying to evaluate resistance, simple comparisons of incidence data from genotypes from different localities, following natural infection or artificial inoculation, are meaningless (McLaren, 1992b) because susceptibility to ergot is extremely sensitive to environmental factors at flowering and a few weeks before (McLaren and Wehner, 1990; 1992; McLaren, 1997; Montes-Belmont et al., 2002b). Cool nights of <12°C at 2-3 weeks before anthesis result in pollen sterility and increased ergot severity. Therefore, tolerance of low, pre-flowering temperatures is important for disease avoidance (McLaren, 1997). Similarly, the mean maximum temperature 1-4 days after pollen shed affects incidence with no disease occurring at >28°C. Interactions between genotype, location and flowering date must be compared by regression analyses because flowering dates of even a day or two apart affect the severity of ergot (McLaren, 1992b; McLaren and Flett, 1998).

Careful screening and selection for floral characteristics that reduce disease severity may prove to be one useful strategy. In Puerto Rico, Dahlberg and Bandyopadhyay (USDA-ARS-TARS, Puerto Rico, personal communication, 1999) found a male-fertile accession with glumes, which tightly clasp the ovary, apparently conferring tolerance to high inoculum loads. This line also showed potential in a male sterile background. In the USA, many A-line sorghums have a protracted stigma receptivity period that confers high ergot susceptibility (Odvody, 1997) and disease reduction may possibly be achieved by decreasing the ergot susceptible period of the A-line stigma. Other advantageous modifications might include reducing the floret gaping period, selecting for more rapid post-fertilization changes in the A-line, breeding for cold temperature tolerance in R-line pollen production and during fertilization, and extending the pollen production period.

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources: