Common Physiological Disorders in Grapevines and Their Remedies

If you’ve ever walked through a vineyard and noticed clusters with shriveled berries, yellowing leaves with green veins, or bunches that seem to have mysteriously lost most of their grapes, you’ve likely witnessed the effects of physiological disorders. Unlike diseases caused by pathogens, these problems arise from environmental stress, nutrient imbalances, or cultural missteps-and they can be just as devastating to your harvest. Understanding what causes these issues and how to prevent them is essential for any grape grower aiming for healthy vines and quality fruit.

What exactly are physiological disorders? 🔗

Physiological disorders in grapevines are non-infectious conditions that disrupt normal plant functions. They don’t spread from vine to vine like a fungal disease would, but they can affect entire vineyard blocks if the underlying cause-be it weather, soil conditions, or management practices-is widespread. The tricky part is that symptoms often mimic those of nutrient deficiencies or even some diseases, making diagnosis challenging without careful observation.

Three of the most common physiological disorders that grape growers encounter are flower bud drop (often called coulure), bunch wilting (or bunch stem necrosis), and interveinal chlorosis. Each has distinct symptoms and causes, but they share a common thread: they all respond well to proactive management and timely intervention.

Flower bud drop and poor fruit set 🔗

Imagine your vines flowering beautifully, only to find weeks later that your clusters are sparse and straggly. This frustrating scenario is often the result of coulure-a French term meaning “shatter”-which refers to the excessive shedding of flower buds and young berries, leaving clusters with disappointingly few grapes.

A related condition called millerandage produces clusters with a mix of normal berries and tiny, seedless “shot berries.” Sometimes called “hens and chickens” because of the size variation, this occurs when flowers are poorly fertilized but remain attached to the cluster instead of dropping off. While millerandage always reduces yield, it doesn’t always hurt wine quality-in fact, some winemakers appreciate the concentrated flavors from smaller berries.

Why does this happen? 🔗

The root cause of both coulure and millerandage is poor fertilization of grape flowers during the critical bloom period. Several factors can trigger this problem. Weather plays a major role-temperatures below 15°C (59°F) can prevent flowers from opening properly, while excessive heat above 32°C (90°F) damages pollen viability. Rain during flowering physically washes away pollen and prevents flower caps from shedding normally.

Vine nutrition matters tremendously too. Deficiencies in boron and zinc are particularly problematic because these minerals are essential for pollen tube growth and fertilization. According to research from Lodi Growers, even when soil and tissue levels appear adequate, the supply of these nutrients coming from the roots is often insufficient to satisfy the high transient demand during bloom.

Vine balance also influences fruit set. Overly vigorous vines divert resources away from developing clusters toward shoot growth, while weak vines simply lack the energy reserves needed for successful fruit set. Some varieties, like Merlot and Malbec, are naturally more prone to poor fruit set than others.

How to improve fruit set 🔗

Prevention starts long before bloom. Ensure vines enter dormancy with adequate soil moisture and maintain consistent irrigation through winter. Prune near budbreak when buds are fully developed. Practice balanced fertility management throughout the season-neither too much nitrogen (which promotes excessive vigor) nor too little.

Foliar applications of phosphorus, boron, zinc, and molybdenum immediately before or during bloom can make a significant difference, especially on soils where these nutrients are limited. Research has shown that molybdenum applications can be nearly as effective as labor-intensive practices like shoot tipping in improving fruit set.

Girdling-the practice of removing a ring of bark from the trunk or cane-can also improve fruit set and berry size. This technique temporarily interrupts the movement of carbohydrates to the roots, concentrating sugars and hormones in the vine parts above the girdle. According to research published in Plant Science, girdling at fruit set consistently increases berry weight by 10-30% depending on crop load.

Bunch wilting and stem necrosis 🔗

Few things are more disheartening than watching perfectly healthy-looking clusters suddenly shrivel and dry up just weeks before harvest. This is the hallmark of bunch stem necrosis (BSN), also known as “waterberry” or “shanking.” The disorder causes necrotic patches to develop on the cluster stems, cutting off water and nutrient supply to the berries.

BSN can strike at different times during the growing season. When it occurs around bloom, it’s called early bunch stem necrosis and typically causes flower buds to drop. When it appears at veraison or later, berries fail to ripen properly, remaining sour and eventually shriveling. The affected portion is usually the tip or shoulder of a cluster, though severe cases can destroy entire bunches.

Understanding the causes 🔗

Unlike fungal diseases, no pathogen causes bunch stem necrosis. Instead, it appears to be triggered by environmental stress that disrupts the vine’s vascular system. Low temperatures and high humidity around bloom can initiate the problem, as can excessive rainfall after veraison. The disorder essentially causes the xylem vessels in the cluster stem to shut down, preventing water transport to the berries.

Nutritional imbalances play a significant role. Magnesium deficiency is particularly associated with bunch stem necrosis, as are imbalances between calcium and potassium. Low nitrogen levels in the vine can also contribute. Vigorous rootstocks that are susceptible to magnesium deficiency may increase the risk.

Management strategies 🔗

Maintaining balanced vine nutrition is your best defense against bunch stem necrosis. Regular soil and tissue testing helps identify potential deficiencies before they become visible problems. If magnesium levels are low, foliar sprays and soil amendments containing magnesium can help prevent BSN.

Managing vine vigor through appropriate pruning, irrigation, and nitrogen management reduces stress on the clusters. Avoid practices that create sudden changes in vine water status-consistent irrigation is better than cycles of drought and excess moisture. Some growers have found success by selecting rootstocks less prone to nutrient uptake problems in their specific soil conditions.

Interveinal chlorosis: when leaves turn yellow 🔗

Walk through almost any vineyard planted on alkaline or limestone-derived soils, and you’ll likely spot the characteristic pattern of interveinal chlorosis-leaves with bright yellow tissue between green veins. This disorder affects the vine’s ability to produce chlorophyll, the green pigment essential for photosynthesis and the plant’s energy production.

According to University of Illinois Extension, chlorosis is most commonly caused by deficiencies in iron, manganese, or zinc. The key to identifying which nutrient is lacking lies in observing which leaves are affected first. Iron chlorosis appears on the youngest leaves at the shoot tips and works inward toward older foliage. Manganese and zinc deficiencies, by contrast, show up first on older leaves and progress outward.

The iron chlorosis puzzle 🔗

Iron deficiency is perhaps the most widespread cause of chlorosis in grapevines, yet it’s rarely due to insufficient iron in the soil. Instead, the problem is usually iron availability. In alkaline soils with pH above 6.5-7.0, iron becomes chemically “locked up” and unavailable for plant uptake-a condition viticulturists call lime-induced chlorosis.

High levels of calcium, phosphorus, or copper in the soil can also tie up iron. The good news is that iron chlorosis often improves naturally as the weather warms and vine uptake increases. However, severe or persistent chlorosis requires intervention.

Correcting nutrient deficiencies 🔗

Foliar applications of chelated iron or other micronutrients offer quick relief but require repeated applications since they only affect leaves present at the time of spraying. For longer-term solutions, rootstock selection is crucial-certain rootstocks like 41B show good tolerance to lime-induced chlorosis and are recommended for calcareous soils.

Soil amendments can help in some situations, though changing soil pH is difficult and slow. Adding organic matter improves soil structure and nutrient availability. In vineyards with chronic chlorosis problems, working with a soil specialist to develop a comprehensive fertility program is worthwhile.

For zinc deficiency, which manifests as mottled chlorosis with unusually small, asymmetrical leaves, foliar zinc applications are effective. Boron deficiency shows as creamy yellow mottles that can lead to poor fruit set and misshapen berries-another reason why pre-bloom foliar nutrition programs are so valuable.

Bringing it all together 🔗

Managing physiological disorders in grapevines isn’t about finding a single magic solution-it’s about creating conditions where vines can thrive. This means paying attention to soil health, maintaining balanced nutrition, managing vine vigor appropriately, and timing your interventions carefully. Regular monitoring throughout the season helps catch problems early when they’re easier to address.

Remember that healthy vines with adequate carbohydrate reserves, balanced nutrition, and appropriate crop loads are far more resilient to environmental stress. Investing time in understanding your specific site conditions-soil type, microclimate, water availability-allows you to anticipate potential problems and prevent them before they affect your harvest.

What do you think? Have you noticed patterns in when physiological disorders appear in your vineyard? What management practices have you found most effective for preventing these issues before they become serious problems?

References

1. https://extension.umd.edu/resource/grape-fruit-set-and-shatter-coulure
2. https://en.wikipedia.org/wiki/Millerandage
3. https://lodigrowers.com/improving-fruit-set/
4. https://www.sciencedirect.com/science/article/abs/pii/S0168945220300972
5. https://grapes.extension.org/bunch-stem-necrosis/
6. https://www.yara.co.uk/crop-nutrition/grapes/nutrient-deficiencies-grapevines/magnesium-deficiency-grapevines/
7. https://extension.illinois.edu/plant-problems/chlorosis