Characteristics of cassava resistance to superelongation disease (Elsinoë brasiliensis)∗

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Abstract
Cassava cultivars were inoculated, under controlled conditons, with single-conidium isolates of E. brasiliensis Bitancourt et Jenkins (Sphaceloma manihoticola Bitancourt et Jenkins) of diverse origins to study cassava resistance to superelongation disease. Disruption of the stem cuticle significantly increased the amount of stem disease on field resistant and field susceptible cultivars. In susceptible and resistant cultivars stem tissue developed a high level of resistance by the age of 10 days. Significant cultivar .times. isolate interactions indicating pathogenic specialization were consistently observed. The interactions were weak, not always reproducible and none were characterized by a hypersensitive-like component. Field resistant and field susceptible cultivars were treated with gibberellin A4 (the plant hormone responsible for shoot elongation) at 0 to 1 .mu.g/ml and inoculated with a virulent isolate of the pathogen to test the possibility of using the hormone as a screening tool. No differences in response to the hormones were detected between resistant and susceptible cultivars. All cultivars showed increases in length at 10-3 .mu.g/.mu.l GA4 and all showed increases in the level of disease (percent diseased area involved in lesion .times. length of affected area) with hormone application. Gibberellin A4 does not appear to have great potential as a screening tool for identifying field resistant cultivars.