Causes of red leaves in the vineyard

Hemant Gohil, District Agent, Gloucester County* Gary C. Pavlis, District Agent, Atlantic County* Daniel Ward, Agricultural Extension Specialist, Pomologist Mizuho Nita , Associate Professor, Plant Pathologist, Virginia State University and Polytechnic Institute, and New Jersey Agricultural Experiment Station and The State University of New Jersey

The appearance of red leaves in a vineyard can be caused by many biotic (viruses, bacteria and fungi) and abiotic factors (nutrient deficiency, cold stress, damage to the root system, etc.). Anything that can cause blockage or stress in the bush's vascular system where water and nutrients are transported can result in red leaves. Although the time of onset, the nature of the appearance of red leaves, and the distribution may vary, the overlap of symptoms with each other, especially the symptoms of red leaves, makes it very difficult to determine the cause. It is impossible to determine what is causing the reddening of the leaves. Based only on visual symptoms. Therefore, the best strategy is not to rely on guesswork, but to get a laboratory analysis of the vine as quickly as possible and get a diagnosis. The following are examples of abiotic and biotic stresses that are often associated with red leaf on grapevines.

Nutrient deficiencies

Figure.1 Potassium deficiency (K), Cabernet Franc variety. Redness of the leaf blade occurs between the veins and remains red between the veins.

Figure 2. Magnesium (Mg) deficiency, Chambourcin variety. Please note that the redness occurs in wedges, starting from the edges of the sheet.

Figure 3. Phosphorus (P) deficiency, Zinfandel variety. Pay attention to the redness from the leaf blade itself and its edges.

Potassium (K) deficiency develops when bushes receive less K than they require for normal growth and development, especially if growing soils have a low soil pH (i.e. pH less than 5 for Vitis vinifera L.), which reduces the availability of potassium to the roots grape bush. Potassium is a mobile nutrient, and when it is deficient, the upper (younger) leaves receive K first, so redness begins on the older, lower leaves first. Symptoms of this redness usually appear at the beginning and end of summer. On red-berry grape varieties, the leaves turn red between the veins, starting at the edges, and as K deficiency worsens, the leaves turn purple (Figure 1). On white-berry grape varieties, the leaves begin to chlorose (become pale yellow or white) due to insufficient chlorophyll production. To monitor plant status for K deficiency, it is recommended to analyze leaf petioles every 2-3 years, ideally during flowering. Target petiole K levels should be 1.5% to 2.5% when sampling at flowering and 1.2% to 2.0% when sampling 70 days after flowering. It is recommended to apply potash fertilizer (KOH) using either foliar or root feeding, depending on the urgency of the action being taken. In case of chronic K deficiency, laboratory tests of the soil should be carried out to determine the level of nutrients and pH in it.

Magnesium (Mg) deficiency: Mg-deficient leaves may be similar to K-deficient leaves; however, with Mg deficiency, leaf redness begins at the edge of the leaf blade, creating a wedge-shaped pattern (Figure 2). Because Mg deficiency is a very serious problem in dark-berry grape varieties, internuclear chlorosis may appear red to brown. In white-berry varieties, chlorosis caused by Mg deficiency remains yellowish. The target level of Mg in leaf petioles should be 0.3% to 0.5% at flowering; or from 0.35 to 0.75% - when sampling 70 days after flowering. Mg sulfate (MgSO4) is used to correct Mg deficiency, but its use is only effective if the soil pH does not require adjustment. If your soil is acidic, apply dolomitized limestone (Mg-containing limestone) as it will also raise the soil pH, improving the rate of Mg uptake.

Phosphorus (P) deficiency : P-deficient leaves of red-berry varieties show transient coloration, while white-berry varieties show chlorosis. Typically, initial chlorosis appears around the edges of the leaves, which turn red in red-berry varieties (Figure 3). Unlike other essential nutrients, P provides greater flexibility in terms of timing of application because it is less mobile in the soil and is not easily removed. The target P level for leaf petioles should be between 0.13 and 0.30%. Monoammonium phosphate (NH4H2PO4) and diammonium phosphate ((NH4)2HP04) are common sources of phosphorus fertilizer. Adjust soil pH if it is below optimal to improve P uptake by roots.

Rubella is the cause of reddening of leaf blades

The main cause of redness in the greenery of the vineyard is rubella. The disease appears at the end of May, beginning of June. The main signs of the disease include the following symptoms:

After infection, spots of a ruby hue (on red varieties), brown (on white varieties) begin to appear on the leaf blade;
The affected leaf blade begins to dry out and fall off without having time to develop;
If the plants begin to bloom, the inflorescences also dry out and fall off. Bunches of grapes lag behind in development.

Diseases

Figure 4. Symptoms of grape leafroll virus, Cabernet Sauvignon. Note the redness of the leaf blade between the veins, but the veins themselves remain green, and the bottom of the leaf looks docked and curled at the edges.

Figure 5. Symptoms of grape leafroll virus, Chardonnay variety. Notice that the leaf blade turns pale green while the veins turn yellowish and the bottom of the leaf appears docked and curled at the edges.

Figure 6. Grape mealybugs (Pseudococcus maritimus) on a grape shoot.

Grapevine viruses , such as leafroll and red spot virus, can also cause leaf redness and result in reduced grape suppression, poor yield and quality, poor vine ripening and early shoot dieback. Since there are currently no effective methods to combat viruses, the only possible method remains prevention - removing infected bushes, planting vineyards with virus-free planting material. Symptoms of leafroll virus usually appear near the beginning of berry ripening and growth as the season progresses. In red-berry varieties, the leaves turn from red to purple in color to the leaf blade, but the veins remain green (Figure 4). In white-berry varieties, the leaf blade becomes pale green and the veins become yellowish (Figure 5). Additionally, docking or downward curling of leaf blade edges is typical of leafroll virus symptoms (Figure 5). Leaf color changes will often develop randomly within the bush. An upward or downward movement of leaf redness will not occur as occurs with some nutrient deficiencies (potassium, magnesium or phosphorus). There are several types of viruses that can cause leafroll disease: lipid-lipid viruses-1 and -3 (GLRaV-1 and -3), which can be spread by mealybugs or other sucking insects (Figure 6). Monitoring the harmful threshold of these vector insects should be done throughout the season (before, during and after flowering), carefully examining the basal area of green shoots and under the bark. Mealybugs can be controlled with insecticides, but recent research has shown that even small numbers of mealybugs can quickly spread leafroll virus in a vineyard.

Figure 7. Symptoms of red spot virus, Merlot variety. Please note that the red pigment appears randomly on the leaf. Reddening of the edges can be mistaken for K deficiency, and downward curling of leaf blade edges can be mistaken for symptoms of leaf curl virus.

Red blotch virus is a recently identified virus that has, however, been around for a long time. As the name suggests, spots of red pigment appear randomly on the leaves of infected grapevines. It can be found at any stage of grape growth in any part of the vine. It can easily be mistaken for potassium deficiency or leafroll virus, especially when leaves appear docked with downturned edges or curled (Figure 7). Due to the recent discovery of this virus, ongoing efforts are being made to understand exactly how and if it spreads in vineyards. It is known that red spot virus can be spread by grafting using infected rootstock and/or scion. For the two viruses mentioned above and any other viruses, the bushes will always be infected. Therefore, when starting a vineyard, it is recommended to obtain virus-tested materials from reputable nurseries. Make sure your nursery has virus testing procedures in place such as the 2010 Protocol. 2010 Protocol materials have been available since 2013, and the availability of virus-free planting material is increasing as more nurseries have adopted the protocol. If you produce your own planting material from your existing vineyards or have obtained planting material from non-certified nurseries, consider sending a representative amount of biomaterial from the vines to a laboratory for virus testing before planting. As a rule, 7-8 young and mature leaves from the bush and 30-50 leaves sample from the vineyard is usually sufficient, but it is better to inquire about the analysis requirements from the virus testing laboratory itself to clarify their specific sampling protocol. When at least one of the vector insects and one of the viruses (GRLaV-1 or -3) are present in your vineyard, vector control will be necessary to keep healthy grape vines free of GLRaV red spot virus.

Figure 8. Symptoms of bacterial cancer at the site of inoculation.

Figure 9. Row of small growths of bacterial canker formed under the bark, Merlot variety.

Figure 10. Leaf redness caused by bacterial canker. Pay attention to the formation of growths of bacterial cancer on the trunk.

Bacterial canker of grapes is caused by the agrobacterium Rhizobium vitis (R. vitis) and is very common in cool climates. Harsh winters or temperature fluctuations in early spring damage the trunk, phloem and vascular tissues, and allow R. vitis to move into grape cells and cause infection. Typical symptoms are the formation of growths on the trunk (Figure 8); however, these growths can also be very small and may form under the bark (Figure 9). Thus, bacterial canker can be mistakenly identified as a nutrient deficiency, since the red leaves thus produced due to stress on the vascular system are easily visible (Figure 10). Other operations, such as mechanical damage to the trunk during weeding or cultivation, can also cause the plant to become infected with R. vitis. As with viral diseases, bushes infected with bacterial canker become ill for life because the bacterium that causes canker is a systemic pathogen. Thus, the fight against bacterial canker in grapes is very difficult. It is necessary to plant vineyards with planting material free from bacterial cancer. Other preventive measures are maintaining bushes with several trunks, so that if one trunk is damaged by a cancerous growth, a replacement can be made without loss of yield, and hilling up the grafting site in the winter months.

What do red leaves on grapes mean?

Grapes cannot be called a capricious crop, but during cultivation, gardeners very often encounter many plant diseases, as well as problems with improper care. As a result, not only the plant itself suffers, but also the grape bunches. The berries become smaller and grow in smaller quantities. In some cases, affected fruits are not allowed for consumption due to fungal infection. Sometimes gardeners note that the leaves of the grapes turn red. The problem requires special attention. It is necessary to find out the cause of the negative phenomenon to prevent loss of crops and plantings in general.

What to do if you suspect bush infection

It is very difficult to pinpoint the cause of red leaves. Tests for viruses (leaf curl virus, red spot virus, etc.) are expensive, so the first recommendation in the identification flowchart is to test the conduction system for integrity and then perform a leaf petiole test (Figure 11). If bole disease and any other damage to the conducting system and nutritional deficiencies are excluded, then samples should be sent for virus testing.

Figure 11: Flowchart of actions to take after you notice red leaves in your vineyard.

*—A county agent is a local official of the USDA Cooperative Extension Service. The agent’s functions include both organizing assistance to farmers at the expense of federal, state and local funds, including university funds, and introducing scientific methods of farming (translator’s note).

Non-infectious rubella

The cause of non-infectious rubella is a deficiency of potassium in the soil layer. If, due to a lack of nutrients, the weather is dry or there are sudden changes in temperature, then the leaves of the grapes turn red. Signs of this phenomenon include the following symptoms:

the spread of the disease is observed immediately throughout the vineyard, and not on individual plants;
redness and spotting appears immediately on all leaves on the vine;
the veins of the leaf plates thicken and become very noticeable against the background of the leaf itself.

The cause of non-infectious rubella can be either poor soil or the presence of clay in it, which retains potassium. Perhaps the bushes do not have a sufficiently developed root system, which cannot provide the ground part with the required amount of nutrients from the deep layers of the soil. If treatment is not started in a timely manner, the plants die due to improper metabolic processes. When the first symptoms are detected, you must immediately spray the leaves with potassium nitrate. The procedure is performed early in the morning or in the evening after sunset. Spraying during the solstice will not give the most positive results. The drug will dry out very quickly and will not have time to exert its effect. To treat non-infectious rubella, five sprays are required, with an interval of 8 days between each procedure. Then, with the onset of autumn, a preparation of potassium sulfate or potassium chloride is added to the soil. Next year it is necessary to perform short pruning of the vines.

Causes of red leaves in the vineyard

Classification by location of injury

Grape diseases can be infectious or non-infectious. Infections are caused by fungi, bacteria, and viruses. Non-infectious diseases occur due to improper care or lack of nitrogen, iron, phosphorus, magnesium, and sulfur.

The most common diseases of grape leaves:

Mildew, or powdery mildew.
Oidium.
Anthracosis.

Reference! All organs of grapes are susceptible to diseases.

Roots

The main disease is root rot. This is a fungal disease; when infected, a powdery coating appears on the roots, wet rot occurs, and roots delaminate. If the infection is severe, the bush may die in two to three years.

Fruit

Diseases that most often affect grape fruits:

Oidium, or powdery mildew. The disease is caused by fungi. Affected fruits crack and then dry out or rot, giving off a rotten fish odor.
Alternaria blight. This is a fungal disease in which grape berries wrinkle, acquiring a silvery sheen.
Bacterial spotting. The disease is caused by bacteria; sunken spots of yellow and brown color appear on the fruits, and the fruits fall off.

Branches

More common on the grape vine are:

Alternaria blight. When infected with fungi, brown or silvery spots appear on the branches.
Black spot, or phallopsis, is caused by fungi. Young shoots are affected, black bumpy dots appear on them, which enlarge and merge into large spots. With black spotting, the frost resistance of the bush decreases and the grapes die when the temperature drops in winter.
Bacterial cancer. Bacteria infect perennial branches, yellowish or brown bubbly growths and light-colored tumors appear on the vine.

In addition to diseases, grape bushes are susceptible to attacks from various pests:

Wasps suck the pulp of the berries, causing great damage to the crop.
Birds love grape berries and peck them, destroying the harvest.
Mice attack young shoots, using them as food in winter.
Small insects and aphids: phylloxera, leaf rollers, scale insects, mites, grape flea larvae, leafhoppers, woodworms and others.

Consequences

Red spots, regardless of the cause of their occurrence, lead to wilting of leaves and early denudation of shoots. A large loss of green mass affects the quality of the fruit. The sugar content decreases sharply, the grapes crumble before ripening and lose their profitable appearance. Disease of the first leaves in early spring stops the development of shoots and prevents full flowering.

A weakened plant subsequently struggles well with other diseases. Mechanical damage to leaves by pests is an “open gate” for serious fungal and viral infections.

Methods for controlling rubella on grapes

If you have determined the reason why the leaves of the grapes turned red, then you need to immediately begin treatment. To control the infection, you must use a fungicide, such as Bordeaux mixture. The course depends on the degree of the disease. Most often, spraying is repeated 7-10 days after the first treatment. In order for the plant to recover from illness, it should be promptly fed with vitamin supplements and minerals.

If something is missing in the feeding, then it is necessary to separately fill the missing component. Why do grape leaves turn red after antiviral treatment? Sometimes one course is not enough for the disease to go away. In some cases, it is necessary to carry out several spraying courses.

Please note that sometimes it happens that the grape leaves turn red not on the top, but on the bottom. The cause of redness of the lower leaves is due to phosphorus deficiency. What to do if you have a phosphorus deficiency? In this case, you should feed the plant with ash or superphosphate. After fertilizing, maintain regular watering and loosening to prevent the soil from becoming moldy.

Infectious rubella

This type of disease does not develop in all shrubs, but locally. The causative agent is a fungus. The spores penetrate the plant's vascular system and begin to weaken it. As a result, grapes cannot absorb nutrients from the soil layer. The plant begins to starve, which is manifested in the appearance of the foliage. Activation of the fungus begins at moderate temperatures and stops when it drops to +13 or rises to +27 degrees Celsius.

To combat the fungus, it is necessary to use drugs intended for the treatment of mildew. Positive results can be achieved by treating the bushes with a 1% solution of beard liquid, 0.4% cuprosan solution, and 0.3% copper oxychloride solution. In some cases, other fungicides also help. Three sprays are performed:

The first - when the shoots reach 25 - 30 cm;
The second is before flowering begins;
Third - immediately after flowering.

Further treatments are carried out every two weeks. Sheet plates are processed on both sides. Spray thoroughly in affected areas. In spring, copper preparations can be replaced with organic fungicides, since copper can cause burns. And in the summer, it is recommended to treat with systemic agents, such as Scora, and then alternate it with Bordeaux mixture.

If the leaves of the maiden grapes turn red, then do not panic. In this case, there is no talk of diseases. The plant variety was bred by breeders to decorate the garden. This braided vine beautifully enriches the walls of a house or gazebo. Typically in the shade, this variety turns red. If vines grow on the southern part of the house, then, as a rule, they remain green. The fruits of such a vineyard are not suitable for food.

If you want to get a high-quality and abundant harvest, you must follow all agrotechnical practices when growing grapes.