What to do when you start seeing disease in the vineyard
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Note: Modified from an article by Alice Wise and Wayne Wilcox, Cornell University
We’ve had quite a variable season so far with rain and sun, humid and dry days after a particularly cool spring. Unusually rainy weather has provided plenty of opportunity for disease to become a problem in vineyards. Black rot, Phomopsis and downy mildew, have already been sighted this year. Our general approach to disease management is to apply preventative fungicide sprays and careful and timely canopy and fruit zone management. But sometimes a situation can get out of control even in the best vineyards, particularly if rain and wind have prevented application of fungicides at critical times. In that case, prepare to treat post-infection if necessary. Several fungicides provide good “post-infection” activity (e.g., the sterol inhibitors such as Elite and Nova). This means that they’ll stop disease development if applied after an infection period has occurred but before disease symptoms appear. However, very few fungicides have the ability to “burn out” (eradicate) active infections once symptoms have become apparent. At most you can expect to knock them back a little bit and suppress sporulation while you keep infections from spreading to healthy clusters and leaves. A few other points: Intervene as soon as possible and make sure coverage is good. If dealing with cluster infections, remove infected clusters (if possible) prior to spraying and leaf pull if time allows. It is unrealistic to expect any material to clean up a raging infection or to adequately penetrate a dense cluster zone. In addition to a post-infection material, remember that continued forward protection is also critical to protect clean fruit.
Not much is known about post-infection activity of fungicides against Phomopsis, as we have not had systemic fungicides that work against this disease until recently and have relied on mancozeb, captan, and ziram to do the job. While Phomopsis, cane and leaf spot are generally not harmful to the vine, fruit infections are and can lead to fruit rot and premature fruit drop as the fruit ripens. If cane and leaf spot infections are noted, it is recommended to protect the fruit and other healthy tissues from infections during rains at least up to bunch closure. With the phosphorous acids (e.g., ProPhyt and Phostrol), we believe we have potential post-infection fungicide options but research needs to be done to show much post-infection activity they actually provide. The strobilurins Abound and Pristine are excellent protectants and may have very limited post-infection activity (approximately 24 hours at most). Because they are systemic and redistribute to some extent in plant tissues, they provide good protection of clusters and leaves.
Both Nova and Elite have excellent post-infection activity and some forward action. The backward control is typically spoken of as extending to about 72 hours, that is, you’ll get control if they’re applied within 72 hours after the start of the infection period (rain). However, in two different field trials (and additional greenhouse experiments), Wilcox obtained good control when these materials were applied even five to eight days after the start of an infection period if excellent spray coverage was provided. This doesn’t mean you should relax after a “black rot rain” if the fruit were unprotected when it started - the sooner you can spray afterwards, the better. However, you’re better off waiting for good spray conditions (within reason) than you are spraying in the wind or rain just to meet an arbitrary 72-hr deadline. Both Nova and Elite have some forward activity, although it’s limited (less than a week); tank-mixing mancozeb or ziram with a post-infection spray of one of these materials will improve the forward activity significantly.
Phosphorous acid products such as Phostrol and ProPhyt provide post-infection control of downy mildew. Good coverage is critical. A dense cluster zone or where foliage is crunched up under bird netting (this is where we’ve had trouble in the research vineyard) can be difficult. Experience with these materials dictates that they are best used early in the infection cycle. Application on an abundance of healthy, sporulating lesions is not the best strategy. Also, phosphorous acid products are prone to resistance development. If downy mildew pressure continues at this high level, alternate phosphorous acids with a product of a different chemistry. Primary options would be Ridomil, copper, mancozeb, captan. Ridomil also provides excellent post-infection activity as well as good forward activity. But using it to try to “burn out” a bad case of active downy mildew is a recipe for resistance development.
There are several options to clean up powdery mildew infections including JMS Stylet Oil; Nutrol (monopotassium phosphate); potassium bicarbonate products like Kaligreen and Armicarb 100; and Oxidate (hydrogen peroxide). Sulfur has good activity against very young infections, but is not great once it’s easy to see that you have a problem to deal with. In Michigan trials, Sulforix (calcium polysulfide) also worked very well as a post-infection contact fungicide, but is assumed to be injurious to sulfur-sensitive grapes. However, none of these materials will clean up and sanitize infected fruit. At best, they will only kill the powdery mildew colonies, leaving scarred fruit but halting the spread of infection to clean fruit. Regardless of strategy, it is probably wise to check fruit closely (look at cluster backsides, clusters jammed up against posts, etc.) shortly after treatment and treat again at the proper interval if powdery mildew infection persists. These materials work strictly by contact, and it’s virtually impossible to contact every square inch of every berry.
JMS Stylet Oil
Of the products listed, only JMS Stylet Oil has provided the best eradication of active infections and is the only material that provides any forward protection. In addition to its post-infection and eradicative activities, the best information available indicates that Stylet Oil provides at least three days, sometimes more, of forward protection under dry weather conditions. However, the oil residue apparently washes off in as little as 1/3 inch of rain, after which most of the protective activity is gone. Thorough coverage is absolutely essential for this or any of the other post-infection post-infection materials to work. Direct spray at the fruit zone with lots of water. Experience dictates that Stylet Oil works if it makes contact with the infected berries. If the clusters are packed in, if leaf pulling hasn’t been done, spray coverage will be compromised and powdery mildew will persist. In the case of JMS Stylet Oil, read the label thoroughly as it is incompatible with a number of key materials including sulfur. Note that JMS Stylet Oil has both a standard and an organic formulation. They differ in the inert ingredients. Also be aware of warnings about application in hot weather (phytotoxicity risk). A study found that Stylet Oil may reduce brix accumulation in fruit when applied multiple times late in the season, so alternating with different fungicides is recommended.
Nutrol, Kaligreen, Armicarb
These are alternatives to when the use of sulfur precludes the use of oil. According to Wilcox, Nutrol, Kaligreen and Armicarb function in the same topical, eradicative, “salt on a slug” mode. Again, these do not provide forward protection and they work best when powdery mildew infection is in the very early stages.
Oxidate in Michigan trials has shown fair to moderate activity against existing powdery mildew infections, but not sufficient to warrant recommendation. The Oxidate label calls for consecutive sprays at 128 fl. oz per 100 gallons and recommends consecutive sprays. Time may be a factor - getting the leaf pulling done and getting consecutive cluster sprays on is time prohibitive for some growers. There have been several questions on tank mixing Oxidate. BioSafe Systems feels that tank mixing Oxidate with either DF or a liquid sulfur should be no problem. To be sure, you might do a jar test first as per the Oxidate label.
Things to remember with respect to post-infection treatments
- Apply treatments as soon as possible after symptoms are seen. (Regular and careful scouting is a prerequisite.)
- If disease symptoms are showing up on leaves and shoots, you can assume that there is plenty of disease pressure to infect the fruit as well.
- Spraying fungicides on raging infections is less effective and can encourage fungicide resistance development in the pathogen.
- Waiting a little longer to ensure good spray conditions is a better option than spraying immediately under poor spray conditions.
- Remove infected clusters (if possible) and leaf pull to ensure good spray coverage of fruit zone and reduce humidity around clusters
- Apply fungicides at the highest labeled rate to ensure good post-infection activity.
- Ensure thorough coverage of leaves and bunches, particularly for contact fungicides.
- Ensure forward protection of healthy plant parts by tank-mixing or applying materials that have good protective activity.
- Always read the label for the pre-harvest interval, incompatibility with other products, and other restrictions..
- Scout again to see if your treatment was effective, keeping in mind that some developing infections may continue to manifest themselves over a couple of days after the spray.