by Sally Colby
“If you’ve been growing grapes for any length of time in this area, you know it’s challenging to control diseases,” said Bryan Hed, research technologist, Lake Erie Grape Research and Extension Center. “This is ground zero for pathogens.” Hed explains that most pathogens are fungal, and many growers are cultivating varieties native to Europe and Asia that didn’t evolve with these pathogens, so they have no resistance.
Although the 2016 crop is in the tanks, isn’t too early for grape growers to think about managing the three most common fungal diseases in 2017, especially when it comes to pruning and dead wood disposal.
The most common early-season issue is phomopsis cane and leaf spot, caused by the fungus Phomopsis viticola. “Typically what we see with cane and leaf spot is black, scabby lesions that tend to form on the first three or four internode regions,” he said. “They’re expanding rapidly in early spring, the inoculant is coming down from the old wood and causing lesions.”
Leaf symptoms include yellowish, pinpoint size lesions that don’t do a lot of damage, but indicate that the pathogen is present. “There’s also a fruit rot symptom with this pathogen, but we don’t see that until ripening,” said Hed. “Infections can get into the cluster rachis very early in the season and move into berries during ripening and cause fruit rot. Infections often remain dormant for months after they get in, which is why early season control is very important.”
The pathogen overwinters on live or dead wood, year-old canes and older cordons. “There are a lot more spores from old wood, particularly from dead wood, so it’s important to control the amount of dead wood. Spores are spread by spring rain. The pathogen can infect tissue at very low temperatures — in the mid 40s.”
Shoots and leaves are highly susceptible to phomopsis infection while expanding early in the season. “The rachises, which is where the money is, are susceptible as soon as you see them, that’s generally two to three inches of shoot growth. If you have problems with phomopsis, you may have to apply sprays very early in the season to control early shoot and rachis infections and prevent fruit rot during ripening.”
Hed says one unique aspect of phomopsis that isn’t seen with powdery mildew or black rot is that current season infections will not produce spores. “That means that the disease works entirely off of overwintering inoculum, inoculum built up from last year and previous years. If you can control or reduce overwintering inoculum, you can go a long way in managing this disease.”
With numerous infection periods in spring, spore sources become “spent.” “Very often, we find that by mid-July, because the current season infections don’t produce any new inoculum, there is no more inoculum, so even though tissue is still susceptible, if you have spores during wetting periods, there won’t be any infection.”
Chemical management options for phomopsis are limited, but are effective and don’t promote resistance. Hed said “Some of the old standards like Captan, Mancozeb and Ziram are relatively inexpensive. Strobilurins are less effective on shoots; better used early in the season around bloom and shortly after to control fruit infections.” He also added that dormant applications of copper and lime sulfur are somewhat effective and can help reduce overwintering inoculum, but they aren’t a substitute for a seasonal spray program.
With all fungal diseases, the immediate pre-bloom and first post-bloom sprays are the most critical for controlling fruit infections. For phomopsis, re-bloom sprays can be applied as early as one-inch shoots. “It’s very important to protect the rachises. They can become infected and you can lose large chunks of those rachises early in the season. Or you can also have fruit rot symptoms develop during ripening.”
Since phomopsis spores overwinter on wood, especially dead wood, cultural management is critical. “Use pruning or training systems that eliminate as much dead wood as possible — training systems that regularly renew the canes. Inoculum can build up on old cordons after many years. The cane system is a better system for controlling phomopsis than a system that maintains a large amount of old wood.” Prunings should be shredded and buried to prevent disease transmission.
Powdery mildew (PM) is found everywhere in the world, no matter what the climate. “The reason is that it doesn’t need free water for most of its life cycle, it affects all green tissue, and can cause considerable damage on vinifera, which is most susceptible. It can attack inflorescences very early and cause a lot of damage. We often see splitting of berries later in the season from infections that took place around pea-size berry size that weren’t obvious. But because the pathogen grows right on the surface, it tends to kill the skin. The skin then cannot expand, and it splits. Split berries going into ripening later in the season leave the fruit open to infection or colonization by a lot of organisms.”
The PM pathogen goes dormant in winter when there is no live grape tissue to live on. Pinpoint-sized chasmothecia are present on tissue in late summer are the dormant, overwintering state. Early spring rains cause the chasmothecia to swell and expel ascospores. Hed says this is the only time this pathogen needs rainfall, and once the spores are released and burst open, the spores can infect leaves whether they’re wet or dry. When late spring/early summer temperatures range between 60 and 80 degrees, conditions are ideal for infection periods without any rainfall at all. The entire life cycle can be completed in less than a week, and the disease can spread rapidly.
Leaves become somewhat more resistant to PM infection as they mature, but protection is still critical to prevent disease. Pre-bloom infections can provide inoculum for developing flowers and fruit. Hed said, “We found that in years when we were out scouting and found early cluster rachis infection, before bloom, that’s a huge red flag that we’ll have potentially big problems controlling mildew, there’s sporulation just millimeters away from fruit, and as soon as the caps come off, the fruit are susceptible and it’s hard to protect them.”
Late season infections can lead to early defoliation and inhibit fruit and cane maturation. “We know that with vinifera in cold climates, it’s important to keep the vine leaves healthy, these are the organs that will be developing the hardiness of the vine going into winter, and we want to maximize that by keeping them clean,” Hed continued.
The good news about PM is that the fruit susceptibility period is extremely short. Even though leaves, rachises and shoot stems might be susceptible all season long, fruit (especially natives) are only susceptible from the time the caps come off and ovaries are exposed until about two to three weeks later. At that point, natives like Concord are almost completely immune, although hybrids and vinifera will continue to be susceptible a little longer; about four weeks post-bloom.
Chemical control can be achieved with strobilurins, but Hed cautions growers to rotate products to avoid resistance. “If you suspect resistance, tank mix with something like sulfur so if it fails, you don’t lost the crop, you’ll still have sulfur to control mildew.” Sterol inhibitors are also effective, but resistance issues are surfacing. More recent chemicals that have been available in the last 10 years or so include quinoxyfen, boscalid, metrafenone, cyflufenamid and fluopyram + tebucanozole. Hed says that a newer product, Approvia® (FRAC 7), should prove to be effective.
Old standards for treating PM such as sulfur, copper and lime are still effective but Hed adds they must be used carefully. “Red hybrids, Concords and red natives are sensitive to sulfur, same with copper. We have to mix lime in to reduce the danger of injuring the vines.” Fertilizer products such as Nutrol® (monopotassium phosphate) are another option, but they only kill what they touch and don’t provide future protection.
Although cultural control for PM is important and can help manage the disease, it isn’t a substitute for fungicides. “Maximizing air circulation, sun exposure, leaf pulling, shoot removal — anything that opens the canopy, if we’re growing vinifera on VSP, we have to shoot trim or summer hedge once or twice during the season. That initiates lateral growth when PM inoculum levels are highest and you get a new buildup of inoculum in the vineyard.” Hed added that good weed control helps keep humidity down in the vineyard, which helps control PM.
Black rot, caused by the fungus Guignardia bidwellii, is easy to diagnose by its characteristic tan lesion on shoots and leaves. “The initial symptom looks like a spot of chocolate milk on a green berry, then it quickly colonizes the fruit,” he said Hed. “The pathogen overwinters on infected fruit, in wood, on the ground and in the trellis. It’s extremely important to get infected mummies out of the trellis at the end of the season during dormant pruning. Getting them out of the vineyard completely will go a long way in relieving the overwintering inoculum load for the following season.”
The primary cycle begins with spring rains splashing spores on tissue. “About two to three weeks after the initial infections, those lesions produce more spores and spread the disease,” said Hed. “A lot of the fruit infection we see is the result of early leaf infections that occur in the cluster zone.”
Hed noted that leaf and shoot tissue is only susceptible when it’s expanding. “Once the leaf becomes fully expanded, it can still become infected, but the infection won’t produce spores,” he said. “That’s why we don’t see leaf infections late in the season.”
However, fruit is susceptible for a much longer time. “Once the caps drop, they’re extremely susceptible, whether they’re natives, hybrids or vinifera,” said Hed. “The natives develop resistance much faster than vinifera. It can take up to eight weeks before fruit of vinifera become fully resistant to this pathogen and may require chemical protection for that long.”
Fungicide products for black rot are highly effective. “Sterol inhibitors and strobilurins are some of the best materials,” said Hed. “They’re rain-fast and provide two or more weeks of protection. The old standards work well too, but remember that in wet seasons, you might need to apply them more often because they wash off.” Sanitation is extremely important –removing and burying mummies. Any management that helps chemicals enter the canopy will also help.