We get a few emails on this subject during the growing season, and especially around planting time. This page is provided to help you identify the cause, but is not intended to substitute for expert diagnosis.
There are many grapevine diseases which have the partial yellowing of leaves as a symptom. Fanleaf virus and Chimeras are two of the most common. Certain insect damage may also cause partial yellowing, or speckling of leaves. But partial is the key word here. A leaf, or multiple leaves that turn uniformly and attractively to a soft yellow color have ceased to produce chlorophyll, but are not likely to be diseased. In a mature vine, more often than not, the root of the problem (no pun here) resides in the area of nutrient deficiency. But on vines young and old, there are several other possible causes for yellow leaves which should be considered.
Another good clue that disease is not involved in the yellowing of a leaf is the yellowing of the leaf stem (petiole). In the case of disease, the petiole remains operative and permits entry of the pathogen into the leaf. Yellowing of the petiole while still attached to a green shoot means that the petiole has failed. The intersection of petiole and shoot may have been damaged and partially dislodged from weather, or other interference.
First year plants which are partially leafed out when planted, may lose one or all of their leaves and the original shoot. Uniform yellowing of leaves will proceed leaf drop. Under normal conditions, the plant will simply start over, and put up one or more new shoots.
The reason for this traumatic start to the new season is the manner in which the plant came to be leafed out in the first place. If started in a greenhouse, or under other protected conditions, the young growth may simply not adapt to "field" conditions. The plant is doing you a favor by shedding this weak growth which, if it survived, would produce a wimpy vine.
Grapevines show signs of nutrient imbalance due to lack of, or excess of nitrogen, phosphorus, potassium, calcium, magnesium, manganese, iron, copper, boron and zinc. Too much, or too little of some of these elements will be immediately apparent in abnormal leaves, or leaf discoloration. Yellowing leaves might mean iron or magnesium deficiency. Yellowing, under these circumstances is not uniform, and not attractive, as described above. In the vineyard, the symptoms may be evident in only one variety, or one location. Application of lime to a vineyard will cause iron deficiency in certain varieties.A Cornell University link on nutrition for vinifera wine grapes is provided below. Much of the general information is applicable to non-vinifera varieties.
Finally, if the leaf yellowing is taking place on a vine in the vicinity of a Black Walnut tree, you will lose that vine and any others that have come into close proximity to the trees root system. See the page on this subject linked below.
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