Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/22/2004
Publication Date: 8/1/2004
Citation: Wanner, L.A. 2004. Field Isolates of Streptomyces differ in pathogenicity and virulence on radish. Plant Disease. 88:785-796. Interpretive Summary: Potato growers observe differences in the incidence and severity of common scab in different potato varieties, different fields, and different years. To understand this variability in scab incidence and severity, pathogenic organisms causing the disease were isolated from scabby potatoes, and their disease-causing potential was examined in radishes. Radishes and other root and tuber crops also get scab, and radishes provide a quicker, cheaper test system than potatoes. The scab-causing organisms produced different numbers and types of scabs on radishes, and some damaged or killed radish seedlings or older radish plants. Molecular features of the disease-causing bacteria were tested, and some lacked characteristic features usually associated with disease. Severity of disease increased as the amount of the pathogenic organisms in the soil increased. These results provide a basis for developing methods for predicting the likelihood of common scab that could be useful for growers and agricultural advisers in better managing scab diseases in potatoes and other tuber and root crops.
Technical Abstract: Common scab is a significant disease of potato, and affects root and tuber crops worldwide. Streptomycetes, a diverse group of soil-inhabiting gram-positive bacteria, cause common scab. To better understand the basis for variability in disease symptoms seen in field situations, streptomycetes were isolated from scabby potatoes. Isolates varied in morphology, pigmentation, and molecular markers of the putative pathogenicity island (PAI). Isolates were evaluated for pathogenicity and virulence in radish. Scab lesions varied in appearance and severity. Disease symptoms also included plant stunting, wilting, necrosis and death. Some pathogenic isolates were missing genes from the PAI; several lacked the nec1 gene, and one was missing the txtA gene encoding thaxtomin biosynthesis, the most reliable pathogenicity determinant. Studies of disease severity over 5 logs initial inoculum density show there is a threshold inoculum density for disease, and severity increases with inoculum density over 3 logs, then reaches a maximum characteristic of individual Streptomyces strains. Lesion severity was not correlated with presence of melanin, the nec1 gene, or whether an isolate reduced seedling emergence or plant survival. Differences in disease symptoms and severity combined with absence of known pathogenicity determinants (txtA) or factors (nec1) suggest there may be pathogenicity factors in addition to thaxtomin.