Investigation of an Outbreak of Fusarium Foot and Fruit Rot of Pumpkin Within the United States

Identity and Pathogenicity of Fungi Associated with Root, Crown, and Vascular Symptoms Related to Winter Squash Yield Decline

Winter squash (Cucurbita maxima cultivar Golden Delicious) produced in Oregon's Willamette Valley for edible seed production has experienced significant yield losses because of a soilborne disease. The symptoms associated with this disease problem include root rot, crown rot, and vascular discoloration in the stems, leading to a severe late season wilt and plant collapse. Through field surveys, Fusarium oxysporum, F. solani, F. culmorum-like fungi, Plectosphaerella cucumerina, and Setophoma terrestris were identified to be associated with diseased tissues, and each produced symptoms of root rot, crown rot, or stem discoloration in preliminary pathogenicity trials. In this study, 219 isolates of these species were characterized by molecular identity analyses using BLAST of the internal transcribed spacer and translation elongation factor 1 alpha genomic regions and by pathogenicity testing in outdoor, large-container trials. Molecular identity analyses confirmed the identity of isolates at 99 to 100% similarity to reference isolates in the database. In pathogenicity experiments, F. solani produced the most severe symptoms, followed by F. culmorum-like fungi, F. oxysporum, P. cucumerina, and S. terrestris. Some treatments of mixed-species inoculum produced symptom severity greater than what was expected from individual species. In particular, the mixture of F. culmorum-like fungi, F. oxysporum, and P. cucumerina and the mixture of F. culmorum-like fungi, F. solani, and S. terrestris had symptom ratings as high as that of F. solani by itself. Results indicate that this soilborne disease is caused primarily by Fusarium solani, but interactions between the complex of F. solani, F. culmorum-like fungi, F. oxysporum, and P. cucumerina can exacerbate disease severity.

Pathogenic, Morphological, and Phylogenetic Characterization of Fusarium solani f. sp. cucurbitae Isolates From Cucurbits in Almería Province, Spain

Fusarium solani f. sp. cucurbitae (syn. Neocosmosporum cucurbitae) is one of the most devastating soilborne pathogens affecting the production of cucurbits worldwide. Since its first detection in Almería Province in Spain in the spring of 2007, it has become one of the main soilborne pathogens affecting zucchini production. It has also been reported on melon, watermelon, and squash rootstocks in Spain, representing a high risk of dissemination in the area. The objectives of this study were to investigate the incidence and distribution of this disease in southeastern Spain and characterize isolates collected over 5 years. These strains were characterized on the basis of greenhouse aggressiveness assays on a range of cucurbit hosts, morphological characteristics, and elongation factor 1-α and RNA polymerase II second largest subunit phylogenies. All pathogenic isolates were highly aggressive on zucchini plants, causing a high mortality rate a few weeks after inoculation. The rest of the cucurbit hosts showed differential susceptibility to the pathogen, with cucumber being the least susceptible. Plants belonging to other families remained asymptomatic. Morphological characterization revealed the formation of verticilate monophialides and chlamydospores forming long chains, characteristics not described for this forma specialis. Phylogenetic studies of both the individual loci and combined datasets revealed that all pathogenic isolates clustered together with strong monophyletic support, nested within clade 3 in the F. solani species complex.

Exploring the evolutionary ecology of fungal endophytes in agricultural systems: using functional traits to reveal mechanisms in community processes

All plants, including crop species, harbor a community of fungal endophyte species, yet we know little about the biotic factors that are important in endophyte community assembly. We suggest that the most direct route to understanding the mechanisms underlying community assembly is through the study of functional trait variation in the host and its fungal consortium. We review studies on crop endophytes that investigate plant and fungal traits likely to be important in endophyte community processes. We focus on approaches that could speed detection of general trends in endophyte community assembly: (i) use of the 'assembly rules' concept to identify specific mechanisms that influence endophyte community dynamics, (ii) measurement of functional trait variation in plants and fungi to better understand endophyte community processes and plant-fungal interactions, and (iii) investigation of microbe-microbe interactions, and fungal traits that mediate them. This approach is well suited for research in agricultural systems, where pair-wise host-fungus interactions and mechanisms of fungal-fungal competition have frequently been described. Areas for consideration include the possibility that human manipulation of crop phenotype and deployment of fungal biocontrol species can significantly influence endophyte community assembly. Evaluation of endophyte assembly rules may help to fine-tune crop management strategies.