The Evolution of a Process for Selecting and Prioritizing Plant Diseases for Recovery Plans.

One element of the cost of dealing with invasive species in the United States is the recovery from the arrival of exotic plant pathogens. We review the development of a process used to prioritize plant diseases for the federally mandated United State Department of Agriculture National Plant Disease Recovery System. A team of university, government, and industry scientists worked together over a 10-year period to develop a science-based objective approach to the challenge of effectively preparing for recovery plans from introduced pathogens, when the timing of the introduction of any single disease is unknown. Over time, the process transitioned from ad hoc, in which recovery plans were written when the relevant experts were able to do so, to a formally organized group-prioritization effort from which emerged the concept of generic recovery plan templates for groups of pathogens and diseases that have similar biological characteristics, and therefore, similar management approaches. Key characteristics for each template were determined through a multivariate analysis for 14 plant diseases for which a recovery plan already existed. The process was validated by a larger group of 15 plant pathologists, for which results were compared with those scored by 14 subject matter experts.

cDNA probes for detection of specific dsRNAs from the fungal pathogen, Monosporascus cannonballus.

Monosporascus cannonballus is an ascomycete fungus that is the causative agent of Monosporascus root rot/vine decline, a serious disease of muskmelon and watermelon. Double-stranded RNA (dsRNA) was identified in approximately 60% of M. cannonballus isolates recovered from infected muskmelon plants in 1993. After repeated laboratory transfer on culture media, the majority of the isolates harboring dsRNAs developed degenerate culture phenotypes and showed reduced virulence (hypovirulence) to muskmelon. Initially, dsRNA purification and cDNA synthesis were attempted in three M. cannonballus isolates harboring dsRNAs. However, numerous difficulties were encountered due to the stable, double-stranded nature of the dsRNAs and contamination of the preparations by fungal rRNA. Several purification and cDNA protocols were evaluated and eventually modified into methods that were ultimately highly effective for cloning dsRNAs from M. cannonballus. The cDNAs derived from purified dsRNA preparations were cloned into a pUC119 plasmid vector and amplified in Escherichia coli. Nine cDNA clones were identified that are specific for medium-sized (ca. 3 kbp) dsRNAs associated with M. cannonballus isolate Ca91-17(96+). The methods used to make the cDNA clones of the dsRNAs in M. cannonballus may be useful for those working on fungal dsRNAs. In addition, these cDNAs may be useful for identifying dsRNAs associated with the hypovirulence phenotype.

The evolutionary biology of Fusarium oxysporum.

Fusarium oxysporum is an anamorphic species that includes both pathogenic and nonpathogenic strains. Plant pathogenic forms cause a wilt disease and are grouped into formae speciales based on their host range; some are further subdivided into pathogenic races. Many formae speciales are comprised of multiple clonal lineages and, in some cases, a pathogenic race is associated with more than one clonal lineage, suggesting independent origins. Although some evidence suggests one pathogenic race may give rise to another, recent derivation of a pathogen from a nonpathogen has not been documented. Most new occurrences of Fusarium wilt appear to be the result of a recent introduction rather than an independent local origin of the pathotype. Asexual propagation is the dominant influence on population structure in F. oxysporum and the absence of sexual reproduction is not likely to prevent this pathogen from continuing to inflict significant damage on susceptible crop hosts.