Characterization of Passalora fulva and tomato leaf mold-associated fungi in Minnesota high-tunnels and the management of common high-tunnel tomato diseases

Abstract

High-tunnels are used in locations with short seasons to extend tomato production both before and after the traditional season as well as to provide protection from extreme temperatures and weather events. However, on average, high-tunnels have increased humidity as well as decreased airflow, which can increase the prevalence of certain tomato diseases. Tomato leaf mold, caused by Passalora fulva, is one disease that is prevalent in the humid high-tunnel environment. Tomato leaf mold lesions contain both P. fulva and other fungal species in the genus Cladosporium. In this research, the diversity of a collection of 93 P. fulva and Cladosporium spp. isolates was assessed by molecular and functional techniques. DNA sequences from the internal transcribed spacer (ITS), β-tubulin, and translation elongation factor-1α (TEF-1α) regions were used to separate P. fulva from Cladosporium spp. and to differentiate species within the Cladosporium genus. Better resolution may be obtained through other techniques such as multilocus sequence analysis or genotype-by-sequencing marker technology. DNA sequences of Avr loci were used to identify mutations within collected P. fulva isolates, relative to corresponding wild-type sequences. A two base-pair deletion was detected in the Avr2 locus, resulting in a presumed non-functional protein. Additionally, synonymous and non-coding mutations were detected in the Avr4E locus and non-coding mutations and a non-synonymous mutation were detected in the Avr9 locus. The temperature growth optima for a selection of isolates were assessed to functionally characterize the predominant species in the collection. In this study, the area of growth varied as a result of incubator temperature, growth media, and isolate. In addition to tomato leaf mold, gray mold and early blight are very prevalent in high-tunnel tomato production. In this research, ten pesticide spray programs were assessed for their efficacy in managing tomato leaf mold, gray mold, and early blight in high-tunnel tomato production, relative to an untreated control. Spray program did not have a significant effect on the incidence and severity of the diseases, the overall health of tomato plants, or the yield. Location had a significant effect on marketable and non-marketable yields. These results may raise concerns about the experimental design of the study, rather than implying that pesticides have no effect on plant health, yield, or disease severity. In the future, experimental plots, laid out across uniform but distinct locations, should be effectively spaced to reduce the chance of pesticide drift between treatments.