Grape Diseases and Bunch Rot

Q. Two years ago I had grape bunches that were shriveling up in the middle of summer while they were still green. I think it was a disease, but it spread and killed off nearly all my grape bunches. How can I prevent this in the future?

Grape diseases can be divided into two main groups; those that attack the fruit and those that attack the vines. This is a bunch rot disease of grapes in Las Vegas. The link is only for Botrytis bunchrot disease. There are several kinds.

A. Grapes don’t typically get borers, but they can get diseases that attack the bunches or young stems. Another disease attacks the new growth. The other problem commonly found on grapes growing in the desert is damage from our intense sunlight.

This was unofficially diagnosed as Eutypa disease on table grapes, a trunk disease of grapes.

            The most common diseases of grape bunches are the so-called powdery and downy mildews. You will not see them, but the disease is flourishing, invisible, right under your nose. This might be a problem later with grapes when there is snow or rain falling on the new growing vines in the spring. It will cause the bunches, all of a sudden, to die green.

During February and March, hope for dry weather when grapes are first starting to grow. If this kind of weather is present then, it’s a good idea to spray a fungicide two or three times during the growing season to prevent bunch diseases like these from becoming a problem. Spray the first dose when bunches are small to protect them from infection. The usual fungicides for homeowners are copper fungicides like Bordeaux. And for heaven sakes, when you are pruning, sanitize your pruning shears

Damage to the bunches or older stems from intense sunlight is not a big problem if grape stems and fruit are grown in the shade of their own leaves.

About the only chemical protectants you can spray as a homeowner are the copper based fungicides, Bordeaux probably leading the group. Remember, fungicides like Bordeaux are protectants and much more effective at protecting new growth from infections. Watch for wet or rainy weather during new growth. If this happens, it is much more likely a disease will occur. Spray with Bordeaux mix after new growth has started and then again as the berries start to enlarge.

Getting Larger Berries in Your Grape Bunches

 

'Flame' table grape growing in Las Vegas.

How do those grape berries get so big in the grocery store? Yes, its true they sometimes use hormone sprays to increase their size but you can do it naturally. Whether you are growing wine grapes or table grapes, it is important to "thin" the fruit to get larger or more flavorful berries. In table grapes the result is larger berries. In most wine grapes, the berries will not necessarily get larger (berry size is ultimately regulated by genetics) but they will get more flavorful.

Here is how.

Fewer Bunches Make Larger Berries

"Balance the fruit load." It is important in wine grapes as well as table grapes. That is a common moniker but what does it mean? It means to reduce the total number of bunches growing in each vine. The minerals taken up by the roots are divided up into fewer bunches and consequently fewer berries. Figure one good sized bunch every 6 to 8 inches.

Reduce Bunch Size

This is done by "pinching" or removing the bottom of the bunch when very young. Sound familiar? You do the same thing to peaches, plums and apples. Then it is called "thinning". The concept is the same. Fewer "mouths to feed" results in larger fruit. The same is true in grape berries. If you reduce the number of bunches AND remove the bottom third of each bunch, the remaining individual berries get larger.

Pinching or removing the bottom third of the grape cluster results in fewer berries and the remaining berries can get larger.

Variety Determines Results

The ultimate berry size is governed by its genetics. The variety 'Italia' is a seeded white grape that grows well in the desert and can be used for fresh eating or making wine. It can get very large berries. While the variety 'Pinot Noir' doesn't seem to get a much larger berry size by thinning but the flavor is more flavorful!

Fertilize and Increase Watering as Grape Growth Appears

Q. I have a grape, with dark colored fruit, growing on my established trellis. It’s already pruned. What should I do to it now?

Fantasy table grape growing in Las Vegas.

A. Of course your grape plant will need water and fertilizer when it starts growing in March. I would apply the fertilizer the first week of March and begin watering weekly during February.

As new growth appears, you should already be watering once a week and have any fertilizer applied. to help push new growth.

Which fertilizer to use? Any fertilizer used for fruit trees will work. It’s applied where the water is delivered to the plant but at least 12 inches away from its trunk. I prefer using a rich compost as a fertilizer instead, but it must be watered beyond any surface mulch so it comes in contact with the moist soil underneath it.

Whether you apply compost or mineral fertilizers to help new grape growth push, do it just before new growth appears for best results.

Grapes grow better in the hot desert if there is “plant litter” (surface mulch) on top of the soil where they receive their water. This surface mulch of 3 to 4 inches deep helps keep the soil surrounding their roots moist and cool. Woodchips from landscape trees make an ideal surface mulch for grapes and fruit trees because they easily rot and decompose into the soil.

Wood, whether its from "forests" or urban forests is a valuable and rare commodity in the desert.

Here woodchips are being dropped at the Ahern Orchard in Las Vegas to be used for soil enhancement around fruit trees and grapes.

It’s helpful to determine if your grape should be “spur pruned” or “cane pruned”. Bunches of grapes are always produced along last year’s growth. The difference when pruning different types of grapes is where the fruit is produced along this new growth. Spur pruned grapes produce their bunches at the base of last year’s growth. Cane pruned grapes produce their bunches from buds further out from this. Buds that produce grape bunches further out, when spur pruned short, are void of grape bunches. Pay attention to your variety of grape and whether it prefers spur or cane pruning.

Grapes and fruit trees grow much better and with fewer problems when NOT grown in bare soil but the soil covered in woodchip mulch.

Simplest way is to delay pruning until you see grape bunches starting to form. I know it can be heart-wrenching, just like thinning peach trees, but removing extra bunches will make each of the berries in the grape clusters larger.

When it’s an unknown grape variety I’m not sure about, I look for old grape clusters, remnants, produced last season. (I will take a few shots of this and insert later. I dont have any shots of this yet.) These can be subtle differences and not obvious to the casual observer. Count back the number of bumps or “buds” where these remnants were attached to older growth. Do this in several locations and get an “average” distance from where it grew. This tells you where you can safely prune last years growth and how much can be removed. Doing this can insinuate if a grape should be spur pruned or cane pruned in the future.

Delay Pruning Grapes in the Desert to Prevent Losses

            

Spur pruning of grape delayed until just before new growth occurs.

Whether you are growing wine grapes or table grapes, it’s a good idea in our climate to delay the final pruning until you are confident freezing, dry and windy weather has passed. The reason for this is to minimize fruit loss because of cold, windy and dry late winter and early spring weather. If you get eager and make your final pruning early, it’s possible that cold, dry windy weather later might eliminate some or all the fruit you intended to keep.

If grape pruning seems a bit daunting, go ahead and prune last years growth long (canes) even if your grape should be spur pruned. As the time approaches for new growth to occur, finish pruning with a second cutting of last years growth to the proper length. This avoids potential losses from the cold and dry winds that can sometimes happen in the desert.

All grape bunches come from buds on last year’s growth. In this way, fruit is produced much like fruit is produced by peaches and nectarines. But this is where the similarity stops. Some grape bunches (Zinfandel, Syrah and Muscat for instance) grow from buds very close to where last year’s growth began. These types of grapes are “spur pruned”; leaving less than an inch of growth remaining from last year.

Leave the canes a bit long and finish pruning when fruit clusters appear to "balance the load".

Other varieties of grapes (Thompson seedless and Concord grapes for instance), their bunches grow from buds several inches further out along last year’s growth. These types of grapes are “cane pruned” because last year’s growth is left longer; 6 to 10 inches in length. This allows the unproductive buds to grow without producing any fruit.

Grape bunches appearing in early May in Las Vegas.

            I delay pruning grapes in our cold desert climate as late as I can. Sometimes even after new growth is well underway. Delaying the pruning this long delays the chance of fruit loss because of freezes and cold drying winds. In the Las Vegas Valley, final pruning may be delayed until late February up until the first week in March. If you want to be doubly sure and a bit lazy about it, delay final pruning until fruit bunches show you where to prune. This might be as late as mid to late April or early May!

You can delay the final pruning of grapes until you actually see fruit clusters (bunches) appearing. Doing this can help balance the load of fruit and make larger berries in the clusters.

            This doesn’t mean you can’t do some pruning earlier. Just be careful and not remove too much of last year’s growth. It’s okay to leave last year’s growth about 12 inches long and cut it shorter than this when you are confident new growth is about to begin or later.

Cancer-Like Growth on Peach Trunk

Q. I have a woody cancerous growth coming from my peach tree trunk. Should I remove it or leave it alone?

Crown gall on peach tree

A. Judging from the pictures you sent, this woody growth looks like the early stage of bacterial crown gall disease. When the tree is older, this cancer-like growth can get quite large but it’s nothing to get overly excited about. It’s not a deadly disease like fire blight can be, but it does cause a gall-like growth of woody tissue near the point of infection.

Crown gall at the actual crown of a tree.

Sometimes it’s seen as a gall on the trunks of infected trees that were “nicked” with a dirty shovel that contacted the soil. In any case, break off this ball from the tree when it gets larger. It will grow back again.

Crown gall on the limb of a peach tree.