Phomopsis cane and leaf spot (PCLS) is a disease of Vitis spp. caused by Phomopsis viticola (Sacc.) Sacc. This disease occurs worldwide but becomes economically important in temperate regions with high atmospheric moisture or rainfall in the spring and early summer. In the presence of conditions favorable for disease development, PCLS has been reported to reduce yields by 30%. All above ground tissues of the grapevine are susceptible during some stage in their development, while rachises and berries remain susceptible throughout the growing season. To increase knowledge on the epidemiology of PCLS, studies were conducted to: 1) transform P. viticola with the gene for production of green fluorescent protein (GFP); 2) observe the temporal pattern of sporulation potential of P. viticola on infected grape shoots, canes, and rachises in vineyards throughout the growing season; and 3) determine the effects of temperature and wetness duration on sporulation rate of the fungus and develop a model for sporulation of P. viticola as a function of environmental variables.
Phomopsis viticola was successfully transformed using two different constructs containing genes for GFP production and hygromycin resistance. Virulence, growth and sporulation rate, and conidial morphology of transformants were similar to the wild type. These transformants will be valuable tools for future etiological and epidemiological studies. Temporal patterns of sporulation potential of P. viticola on infected grapevine tissues corroborate that PCLS is a monocyclic disease, with sporulation occurring in the year following infection. Sporulation had the potential to occur approximately from bud break until shortly after bloom. There was a consistent temporal pattern to sporulation potential across sampled vineyards, years, and grape tissues (rachises and canes), described by a modified beta model, with peak relative sporulation potential occurring around 10 May (Eichhorn-Lorenz stage 12: five to six leaves unfolded with inflorescences clearly visible).
Lower and upper temperature limits of sporulation of P. viticola on infected grape canes were found to be near 5 and 35°C, respectively. Optimum sporulation was near 21°C, and sporulation increased monotonically with increasing wetness duration at all temperatures. A nonlinear generalized Analytis Beta model accurately described sporulation rate from controlled environment studies. Moreover, interruption in wetness disrupted spore production, and sporulation rate declined with increasing interrupted wetness duration (IWD). An IWD of 8 h or more resulted in significantly and substantially less sporulation compared to the control (0 h IWD) (P < 0.01). The time trend in spore density of P. viticola for each rain episode in a ‘Catawba’ vineyard over 3 years could be predicted based on the controlled-environment-based Beta model, as expanded with terms for dispersal from rain (based on the amount of rain in mm) and time of season (reflecting the aging of the lesions). Absolute density of spores in a vineyard could not be predicted because the total density of lesions is not known. Results can be used to improve a disease warning (forecasting) system for PCLS, by incorporating a prediction of relative inoculum density in vineyards following rain events.