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  • Author or Editor: Raymond Hammerschmidt x
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Systemic resistance to necrotic lesion forming pathogens can be induced in certain plant species by inoculating a young leaf with a limited amount of pathogen or by treating with specific non-pesticidal chemical compounds. A physiological change correlated with the induced resistance response is an increase in the activity of acidic apoplastic peroxidases. When seedlings of 17 inbred lines of fresh market and pickling cucumbers were foliar treated with 20 ppm 2,6-dichloroisonicotinic acid (Ciba Geigy 41396) and subsequently inoculated with either Pseudomonas syringae pv. lachrymans or Colletotrichum lagenarium, significant differences were observed in the number of lesions that developed. CG 41396 treatment also gave rise to 4-fold (Producer and Early Russian), 3-fold (Poinsett and Straight 8) and 2-fold (Delcrow, WI 2757, TMG-1, TG 72) increases in peroxidase activity within inbred lines. Distinct changes in acid peroxidase electrophoretic isozyme banding patterns were observed within certain inbred lines after treatment with CG 41396. These results indicate that genetic variability exists within Cucumis sativus with respect to plant response to physiological disease resistance inducing treatments.

Free access

Armillaria root rot (ARR), caused by Armillaria species and Desarmillaria tabescens, is a severe disease that affects stone fruit trees in the United States. One strategy to mitigate the impact of this disease is to develop ARR-resistant rootstocks. However, current techniques to screen Prunus species for resistance to ARR are time-consuming, labor-intensive, and may not fully replicate field conditions. To address these limitations, we developed a new rapid in vitro screening assay, which uses roots of 2-year-old Prunus rootstock genotypes. We screened 12 Prunus genotypes against Armillaria mellea, Armillaria solidipes, and Desarmillaria tabescens in vitro. Freshly excavated root segments were placed next to or on top of fungal cultures. After 21 days, the circumferential percentage and horizontal length of the fungal colonization and the ability of the fungus to enter through root periderm were evaluated. The root tissue surrounding the infection was also evaluated to assess any response reactions against the ARR pathogens. Our results showed that inoculated root tissues displayed signs of fungal infection, and infection and host responses varied among the Prunus genotypes. Host responses similar to those observed in the field, such as compartmentalization of infected tissue with barrier zones, necrophylactic periderm formation, and callus formation on root surfaces, were observed and were more evident in less susceptible genotypes. In conclusion, our newly developed assay, which uses freshly excavated roots from 2-year-old rootstocks, can rapidly screen Prunus genotypes for resistance to ARR.

Open Access

The Oomycete plant pathogen, Phytophthora capsici, causes root, crown, and fruit rot of winter squash (Cucurbita moschata) and limits production. Some C. moschata cultivars develop age-related resistance (ARR), whereby fruit develop resistance to P. capsici 14 to 21 days postpollination (DPP) because of thickened exocarp; however, wounding negates ARR. We uncovered the genetic mechanisms of ARR of two C. moschata cultivars, Chieftain and Dickenson Field, that exhibit ARR at 14 and 21 DPP, respectively, using RNA sequencing. The sequencing was conducted using RNA samples from ‘Chieftain’ and ‘Dickenson Field’ fruit at 7, 10, 14, and 21 DPP. A differential expression and subsequent gene set enrichment analysis revealed an overrepresentation of upregulated genes in functional categories relevant to cell wall structure biosynthesis, cell wall modification/organization, transcription regulation, and metabolic processes. A pathway enrichment analysis detected upregulated genes in cutin, suberin monomer, and phenylpropanoid biosynthetic pathways. A further analysis of the expression profile of genes in those pathways revealed upregulation of genes in monolignol biosynthesis and lignin polymerization in the resistant fruit peel. Our findings suggest a shift in gene expression toward the physical strengthening of the cell wall associated with ARR to P. capsici. These findings provide candidate genes for developing Cucurbita cultivars with resistance to P. capsici and improve fruit rot management in Cucurbita species.

Open Access