Protecting grapevines from Phomopsis Cane and Leaf Spot

With the prediction of rain next Monday (April 6th), Valley growers should consider applying a fungicide to protect young succulent growth from Phomopsis infections. Spring showers create optimal conditions for Phomopsis viticola pycnidia to begin producing spores that can easily spread and infect susceptible cultivars such as Thompson Seedless, Flame Seedless, Redglobe and Grenache. Timely fungicide applications are paramount in managing this disease when spring rains are predicted.

 

Symptoms first appear on leaf blades as tiny dark brown-black spots with yellow margins (Fig. 1). Under optimal environmental conditions, spots may coalesce and kill portions of expanding leaves. Severely infected leaves become distorted over time, die and drop from the canopy. Tender green shoots that become infected display tiny black spots near the base of the shoot, which also begin to merge. Heavy infections result in blackened tissue that cracks and displays a scarred, scabby appearance at the basal end of the shoot (Fig. 2). Early, heavy infections will stunt shoot growth and development. Some buds may not emerge at all due to bud mortality. Thompson Seedless and Redglobe may experience poor budbreak after a season of heavy Phomopsis viticola infection. When shoots have developed 12-24” of growth, shoot breakage may occur during strong winds when heavy infections are present.

 

Favorable conditions include temperatures between 60-75ºF and free moisture or humidity near 100%. Extended periods of rain will increase infection and may make it difficult to apply fungicides. Thus, it is important to protect young green shoots prior to forecasted rain. Spores are released in large quantities from pycnidia found on diseased wood at budbreak and splashed by rain onto the young growing shoots. Once growth has reached 15-18 inches the canopy forms an umbrella and helps prevent the splashing effect, therefore decreasing the need for fungicide applications.

 

Registered fungicides applied from budbreak through 12” of growth (mid March-May) act as foliar protectants and need to be applied to the foliage after budbreak (during the highest potential infection period). Foliar applications need to be applied prior to rain events in order to be effective and will need to be reapplied after significant precipitation.

 

Various copper/sulfur materials (alone or in combination), ziram, captan, mancozeb or maneb are all foliar contact protectants and need to be applied before spring rains. Additional applications will be needed after significant rainfall or on new growth that was not present during the initial application. Fungicides from this group should be considered for early season management due to canopy size and costs.

 

The class of fungicides known as the strobilurins (Abound^®, Sorvan^®, Flint^® and Pristine^®) are foliar systemics and sustain activity throughout long rain events. Unlike the contact protectants, they have the advantage of not being washed off during a rain event. These fungicides are also effective against grapevine powdery mildew. Care should be taken to rotate the strobilurins with fungicides that are multiple site inhibitors. As the canopy continues to grow (4-6”), these fungicides should be considered as replacements for the non-systemic fungicides. Their efficacy during storms lasting more than a day will help protect new susceptible tissue.