Aug 28, 2014
Tackling phomopsis crucial for vineyards

Whether it’s battling phomopsis or other diseases, taking a cost-effective approach in designing a fungicide program is essential for vineyard management.

“Consider not just the cost of the (fungicide) product but also the labor, fuel and a cost in terms of compaction of vineyard soil if you drive over it too often, “ said Annemiek Schilder, a plant pathologist at Michigan State University (MSU).

“You also want to reduce the number of trips through the vineyard if you are able to,” Schilder told grape growers in July, at the 25th annual Viticulture Field Day at MSU’s Southwest Michigan Research and Extension Center.

Schilder said various cultivars have different tolerance to disease pressure.

“Minnesota varieties are more disease resistant. Especially Marquette, for instance, has powdery mildew and downy mildew resistance. However, it is susceptible to anthracnose. It’s manageable, but you have to be diligent to get it out of the crop. It presents as spots on berries, leaves and canes.”

“A lot of the hybrids tend to be more resistant, so you may not want to spray every time – maybe every other, and concentrate on wine grapes. Make sure the highly disease-susceptible varieties are the ones that are getting most of your attention.

“I would really focus on doing dormant sprays. Put on one or two dormant sprays for phomopsis as needed, depending on the variety. Then I would really focus on the developing fruit. I would put on spray at immediate pre-bloom and first or second post-bloom with some of the stronger products, the best products that I could get, and really protect that fruit regardless of the conditions, because most years there is disease or disease pressure, so I would protect that fruit.”

She said it’s important to “lay off of my fungicide program, and scout and just look at the foliage. I believe if you are scouting frequently every week then you can nip some of these diseases in the bud. If you start to see downy mildew, we have products that can knock it out. You know it’s there and can be preventive. Most of the vines will have enough foliage. If you keep it on you have enough to ripen fruit.”

Due to early rain and cool weather, downy mildew is a “big threat this year. Usually, we see it in July, but not to this extent. If you see it early, you can control it.”

Phomopsis

Schilder called phomopsis a “main disease, especially in Niagara and Concord, and also in wine grapes.”

“Sometimes by end of season it’s very difficult to distinguish. A lot of people might think they have botrytis, but it’s actually phomopsis. The last few years we’ve seen late black rot at the same time as early phomopsis symptoms. If you see rot (at the end of July, early August), it’s black rot. Phomopsis doesn’t start to show until about two weeks before harvest.”

The main symptoms of phomopsis are leaf spots – little yellow spots with brown centers or spots or streaks on the canes, she said.

“The main reason we have so much phomopsis now versus maybe a few years ago is there’s a lot of old wood left in the vine through mechanical pruning. Mechanical pruning and no follow-up leaves a lot of old wood, and that’s where the fungus overwinters.

“We’ve found if you do an umbrella kniffin system, where you don’ leave much old wood at all, you will have very little disease.”

The umbrella kniffin system is established by bringing the trunk up to the top wire and leaving four or more canes, bearing a final total of 50 to 60 buds near the top of the trunk (head). Remove all other wood except two renewal spurs (short canes of two buds) near the head. After adjusting the number of buds, retie the trunk. Then, bend the canes rather sharply over the top wire so the outer bark cracks, and tie the tips to the bottom wire.

The renewal buds will develop into shoots that probably will not be fruitful but should be allowed to grow. They are there to be used, if necessary, for retaining canes the next year. The buds on the four or more canes will form fruiting shoots that do not need to be tied because the vine already has been trained. Some of those shoots probably will be well located and can replace the original canes the following season, in which case the renewal shoots are not needed.

Growers tend to compensate in very dense canopies impacted by hard winters and spring frost by leaving extra wood and buds “just in case.”

“If you don’t follow up you have a very dense canopy that is very difficult to penetrate with a fungicide sprayer, so you end up having more wood, and where there’s so much inoculum these days (it leads to) poorer coverage. Some cutting back in the volume of the sprayer just doesn’t cut it, especially if you are having high phomopsis pressure. You just need to really buckle down and have a few good sprays.”

Sprayer strategies

Proper sprayer choice and calibration is crucial, Schilder said.

“With a lot of airblast sprayers, calibration is important. You are trying to spray just the fruit zone. Reduce the spray to the fruit zone and not at all of the foliage. Increasing the spray volume – mainly for airblast sprayers – is effective. Giving it 50 gallons per acre is better than 20, especially when you are looking at after bloom. As the canopy increases, you might increase to 75 or even 100 (gallons per acre).

“Look at the coverage and whether you are getting any runoff. Any runoff is lost. You want good coverage but not too much.”

She said some of the newer power sprayers should run at a lower volume.

“In a study of blueberries, we increased the volume on some of those sprayers and we actually got worse control. It depends on the sprayer – some have excellent control.”

Other factors that can be used to enhance spray effectiveness include:

Driving every row at a slower speed to get the best out of the fungicide.

Consider the weather conditions to avoid rain washoff, selecting periods where the fungicide will dry rapidly on the leaves, allowing for a good uptake of the chemical.

Sanitation: Managing for less wood produces less disease.

Scouting and disease assessment. Frequent scouting can help assess disease pressure and also the efficacy of sprays being applied.

Gary Pullano




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