By Certis Biologicals
Grape crops face a variety of diseases that can appear due to seasonal and environmental changes, and understanding the symptoms can help growers to stop disease pressure before it affects crop yield.
Let’s dig into root causes of fungal diseases in grapes, how you can solution for them using prevention and control, and how all of these measures will enable you to grow healthier grapevines, strengthen the health of crops, and increase yield in the long run.
Downy mildew is caused by the fungus Plasmopara viticola, which attacks all green parts of the vine, specifically targeting the leaves.
- The main symptoms include yellow to reddish brown, with angular or oily lesions.
- The fungus sporulates and appears as a delicate and dense white and cotton-like growth in the lesions, which causes infected shoot tips to thicken and curl into white sporulation, which eventually causes the leaves to turn brown and die.
In grapes, young berries are highly susceptible to powdery rot, and the fruit will appear gray when infected (called gray rot), and then become covered in a “downy” felt of fungus spores.
Downy mildew should be treated with a preventative product such as LifeGard® WG, and then grapevines can be sprayed with an effective fungicide just before blossoms open, and then 7-10 days later, followed by intervals of 10-14 days and a final application a few weeks later, depending upon label instructions and disease pressure.
The fungus Uncinula necator causes powdery mildew, which has a narrow host range and attacks only grape crops and a few closely related species.
- The early signs include chlorotic spots on the upper leaf surface, and later as webby, white mycelium on the lower leaf surface.
- The fungus then takes on a “powdery” or dusty-like appearance that may take over the entire surface of the berry.
- On mature fruit, it appears as black or brown web scarring, and other symptoms may include red blotchy areas.
Preventative treatment will help prevent diseases as the crops grow, and fungicide treatments should begin promptly and repeated at appropriate intervals.
Bunch Rot / Botrytis
Bunch rot comes from the fungus Botrytis cinerea, and can occur anytime during the growing season due to common environmental causes, and it can overtake grape crops quickly.
- Early season shoot blight may occur prior to fruit ripening following prolonged warm, moist weather typically caused by spring rains.
- The fungus then causes patches of soft brown tissue, which kills the infected plant part.
- Individual berries can become infected due to infections from leaf axils, and then clusters can turn brown or reddish.
- Typical signs show epidermal cracks that form, and fungal growth produces mycelium and spores that results in a gray, velvet-like appearance of the berries.
Practice canopy management such as shoot thinning, hedging, and leaf removal of basal leaves to reduce incidence and severity of disease, but do not remove excessive leaves in warmer growing areas, as this can lead to sunburned fruit.
A fungicide will be more effective with good coverage, which is also affected by canopy and growth. Apply sprays before rainfall, at bloom, or at veraison. It’s important also to control insect populations, as entry wounds from their feeding promote Botrytis infections.
Leafy Spot / Phomopsis
Leafy spot, or leafspot, is caused by the fungus Phomopsis viticola that was formerly known as “dead-arm.”
- Early signs show small dark spots and yellowing on the edges of leaf blades and veins.
- Spots typically occur 3-4 weeks following a rain because moisture is required for infection, and leaves may die if too many spots build up.
- If basal leaves develop heavy infection, they may become distorted and never fully develop in size.
- Spots can also occur on the shoot and cause the shoots to crack and infect further, showing a scabby appearance, which can move to the flower cluster stems.
- Summer heat diminishes lesions, but rain can cause spots to occur on clean berries, which causes them to shrivel and mummify.
Be on guard to find the presence of lesions on spurs and canes in areas of the vineyard that might be exhibiting poor budbreak. Liquid lime sulfur can help at 10gallons/acre in 100 gallons of water before rainfall in winter to reduce overwintering Botrytis or powdery mildew spores to grow.
If disease symptoms show in spring, use foliar treatments if rainfall is predicted after budbreak. Some contact products may need to be reapplied after significant rainfall to protect shoots up to 18 inches in length.
Black Rot / Xanthomonas ssp
Black rot can be one of the most damaging diseases in grapes if it is not managed early in the season. It is caused by the fungus Guignardia bidwellii and cam infect leaves, shoots, berries, tendrils, cluster stems, and more in grapes.
- Disease development favors warm, humid weather and symptoms first appear as small yellow spots on plant leaves.
- The spots become lesions, which have a dark reddish-brown border and tan or brown centers.
- Eventually, the spots become black in the lesion in a ring pattern, and the spores from these infect new plant tissue, thus spreading the disease when temperatures are warm.
- Fruit symptoms on grapes typically appear after the berries are larger than a pea size, when brown spots appear and then the fruit shrivels and turns black into a raisin-like appearance (fruit “mummies” which develop new spores that can spread through the air into new tissues).
Sanitation plays an important role in the control of black rot, so be sure to check the leaves throughout the season to prune leaves with lesions in the spring if they appear, because they will cause further infection if they are not removed. Also, check the vineyard ground for mummies to ensure that the disease does not get carried through the air to infect other vines or plants.
Use preventative measures to control disease, and fungicides can be sprayed during the period of early bloom through 3-4 weeks after initial bloom.
Shoot Blight / Sclerotinia spp.
Shoot blight occurs in grapevines from the fungus Sclerotinia sclerotiorum, which is a fungal pathogen of 400+ plant species that feeds on dead tissue and thrives when rainfall and cool weather occur in early spring.
- The disease can reduce yield by damaging or killing fruit-bearing shoots in the current season and in future years, and it can often be mistaken for Botrytis shoot infection in the early stages.
- It initially appears as light brown lesions on the shoot and develops at the shoot base or at a node.
- Symptoms develop into long cankered lesions surrounded by water-soaked tissues at the base of new shoots, and infection travels down the stems, making them break easily when bunches begin to develop.
- Distal shoots may wither, appear with white fungal growth after moist, warm conditions, and can form black sclerotes on the outside of the shoot.
- The disease mainly infects the shoots, but can spread to the berries.
Warm conditions slow the spread of Sclerotinia shoot infection. If these symptoms occur in spring, the disease is unlikely to cause significant yield loss, but preventative measures should be taken to ensure that the berries stay disease-free throughout the growing season.
Bacterial blight in grapevines is caused by the bacterium Xylophilus ampelinus which survives in the vascular tissues of infected plants. It is a highly damaging systemic disease that affects commercial cultivars.
- It affects the leaf, petiole, stem, root, shoot, or flowers.
- Symptoms include linear reddish-brown streaks that expand upwards on the shoot.
- The streaks then darken, crack, and develop into cankers.
Understanding the symptoms will help you be able to treat disease pressure as it arises, but it’s also important to put preventative methods into practice throughout the growing season to decrease disease pressure in grapes.
Disease-causing fungi can rapidly colonize a plant surface, so preventive fungicides are effective prior to the onset of diseases. Preventive fungicides have an upfront cost, but they also reduce the incidence of disease and minimize the need for costly post-infection practices after plants have been infected, which can also lead to greater loss in yield and profits.
LifeGard® WG is a biological stimulator that induces a natural immune response in the plant that reduces the occurrence and severity of disease. Double Nickel® LC is a technologically advanced broad spectrum biological fungicide and bactericide that can control or suppress over 90 different fungal and bacterial diseases and is tested to be effective on grapes. These products are ideal for organic production or in an integrated disease management program.
Learn more about how to combat diseases and other pests in grapes at our Grape Portfolio.