Relationships between incidence and severity of Stewart's and Goss's bacterial wilts and yield of sweet corn hybrids

Abstract

Relationships between severity and incidence of Stewart's and Goss's bacterial wilts (caused by Erwinia stewartii [Smith] Dye and Clavibacter michiganese subsp. nebraskense [Schuster] Davis et al.) and sweet corn ( Zea mays L.) yield were studied in factorial experiments with sweet corn hybrids, pathogens and incidence of initial infection as main factors. Relationships were similar for the two bacterial diseases but were affected differently by host resistance. Relationships between disease severity subjected to the square root transformation (severity 1 2 ) and incidence were linear in resistant and moderately resistant hybrids, cvs Miracle and Gold Cup, because the bacterial wilts did not become systemic in these genotypes following inoculation at the five- to seven-leaf stage. In the susceptible hybrid, cv. Jubilee, the relationship between severity 1 2 and incidence was curvilinear, with severity ranging from about 25% to 65% at 100% incidence, depending on the host growth stage at which plants were inoculated. Yield of the resistant and moderately resistant hybrids was not significantly affected by inoculation at the five- to seven-leaf stage. In the susceptible hybrid, ear weight and number of marketable ears were reduced approximately 1·5% for each 10% increase in incidence when plants were inoculated at the five- to seven-leaf stage in 1985 and approximately 5% and 3%, respectively, for each 10% increase in incidence when plants were inoculated at the three- to five-leaf stage in 1986. In 1988, relationships between yield and incidence were confounded by secondary spread; however, significant relationships between yield and disease severity at harvest were found in the susceptible hybrid inoculated at the three- to five- or the five- to seven-leaf stages. Estimating Stewart's and Goss's wilt severity and yield reductions based on incidence of initial infection could be of practical importance because it simplifies the method of disease assessment; however, estimates of severity and yield reductions must consider the level of host resistance, the growth stage at which plants were infected and the potential for secondary spread due to alloinfection.