Plant Disease Symptomatology: Cucumber Green Mottle Mosaic Virus (CGMMV)-Infected Cucumber Plants Exposed to Fluctuating Extreme Temperatures

Greenhouse grown cucumber plants inspected during and following extreme variations in environmental temperatures showed new characteristics of Cucumber green mottle mosaic virus (CGMMV) disease manifestations. An increasing occurrence of high temperature-induced CGMMV disease recovery has been associated with a new phenotype identified at early stages of a reemerging disease. Symptoms of bright yellow islands (BYIs) conspicuous amid a dark green surrounding (DGS) tissue were detected in up to 10% of the symptomatic plants in commercial net-houses that showed 50-60% recovery following an extreme temperature wave. The high CGMMV titer in the BYIs and the low virus titer in the associated DGS tissue were infectious in biological assays. A controlled study of CGMMV-infected cucumber plants subjected to an abrupt temperature raise, from 25°C to 32°C, allowed monitoring symptomatology of early stages of disease recovery and reemergence. Our study revealed an early and late BYI stages that were associated with a consecutive up- and down-regulation of jasmonic acid (JA) signaling and a corresponding increase in JA inhibitory pathways concomitant with disease progression. Interestingly, the cucumber plant response to the late disease stage was associated with increasing activity of the phenylpropanoid pathway in the DGS tissues apparently indicating high activity of plant resistance pathways towards the disease including levels of reactive oxygen species and specific interference with tobamoviruses. Disclosing the cucumber molecular pathways, engaged at the early CGMMV disease initiation stages, have allowed pinpointing the preferable treatments in order to confine the CGMMV disease and prevent viral systemic infection.