Contribution of Alternative Weed Hosts and Agronomic Practices to the Spread of Goss\u27s Wilt in Indiana

Goss’s wilt is a bacterial disease found in corn caused by the bacterium, Clavibacter michiganensis subsp. nebraskensis (Cmn). This disease was first discovered in 1969 in Dawson county Nebraska and has since been confirmed in thirteen states in the United States and three provinces in Canada. Factors that contribute to the spread of Goss’s wilt are infected corn residue, no-till, continuous corn, storms, infected seeds, and alternative hosts. Infected residue is the primary inoculum source for Goss’s wilt because the bacteria requires either a living hosts or residue to survive. Practices such as no-till and continuous corn can leave large amounts of corn residue on the soil surface and potentially influence the inoculum potential of Cmn. Early theories on Cmn infection assumed that wounds created from storms were initially required for infection to occur. Recent research has revealed that Cmn can infect corn without wounds, but storms can still aid the bacteria by moving it from the soil surface and onto lower portions of the plant. These factors are helpful in allowing Cmn to move short distances and build up the inoculum potential in infected fields. However, long distance movement of Cmn is limited but possible through infected seeds. Seeds infected with Cmn is rare but researchers have demonstrated the ability of Cmn to infect kernels and an even lower percentage of infected seed can transmit Cmn from the seed to the seedling. This gives Cmn the ability to travel longer distances but is less likely to occur. Another source of inoculum and possibly the least understood factor potentially contributing to the movement and inoculum potential of Cmn are the known alternative hosts. Some grass species have been found to be hosts for Cmn. Many of these hosts are considered weeds and can be found in production fields potentially serving as a bridge hosts in non-hosts crops. The cover crop annual ryegrass is the only alternative hosts utilized as a cover crop. When using annual ryegrass as a cover crop living tissue is available for Cmn to overwinter in. The objective of this research was to evaluate the susceptibility of alternative hosts when inoculated with Cmn, factors contributing to the spread and persistence of Goss’s wilt in Indiana, and the influence a susceptible cover crop could have on the overwintering of Cmn and corn yields. In experiments evaluating alternative hosts for their susceptibility to Cmn, early studies used a 1 x 106 or 1 x 107 Colony Forming Units (CFU) per ml inoculum concentrations to screen weeds when determining alternative hosts for Cmn. Only two studies had gone further and compared alternative hosts for their level of susceptibility to that of a corn hybrid at a single higher inoculum concentration. Our greenhouse experiment evaluated annual ryegrass, giant foxtail, and johnsongrass for their level of susceptibility compared to a susceptible and moderately resistant corn hybrid with a tenfold dilution ranging from 1 x 100 to a 1 x 107 CFU per ml inoculum concentrations. Days required for initial lesion development to occur was also recorded and compared amongst the different hosts and inoculum concentrations. The two corn hybrids had higher disease severity than the alternative hosts. The two corn hybrids also had more disease severity as inoculum concentrations increased, while alternative hosts showed no difference between the different inoculum concentrations. Hosts required similar number of days for initial lesions to develop, except annual ryegrass was a few days slower to develop lesions. Hosts also required fewer days for lesion development to begin as inoculum concentrations increased. In conclusion, corn is a better host for Cmn and contributes more to the inoculum potential. Alternative hosts however, still had lesion development at lower inoculum concentrations, and may be able to serve as a refuge in non-hosts crops. (Abstract shortened by ProQuest.