Barbara’s VC News Article With Good Information for Now
Rain brings plenty of headaches
As of early August, North Carolina’s peanut crop was looking good and excellent yields were projected. That being said it has been a very busy disease year so far, and as things are shaping up, there is more to come.
With the exception of spotted wilt and nematodes, every important peanut disease is caused by one species or another of fungus, and fungi like nothing better than lots of moisture and mild temperatures, exactly like the weather we have had so far this summer. Not surprisingly, leaf spot advisories have been very active this year, and Sclerotinia has been off the charts. If the weather stays cool, it’s possible that we will not see much stem rot this year, but we may see some Rhizoctonia limb rot moving in instead. All in all, it will be vital to maintain good leaf spot control in August and early September. As always, spray programs should include at least one application of a fungicide that controls soil borne diseases, since these fungicides will keep both stem rot and Rhizoctonia limb rot in check.
This has been a year for Sclerotinia we’d all like to forget. As I write this, we are getting reports that Omega is in very short supply and growers are looking for alternatives. The 1.5 pint per acre rate of Omega is the standard for Sclerotinia blight control and is very effective if applied at the correct time. Careful scouting and the Sclerotinia advisories will help you to identify when to spray. In lower pressure situations, one pint per acre of Omega can be very effective. Using this lower rate may also help growers stretch their supplies, allowing a follow-up application later. There is a strong response to rate, however, so use 1.5 pints per acre in challenging situations. As we move into September, remember that Omega has a PHI of 30 days from last application to threshing.
Alternatives to Omega include Endura, which also is in very short supply, and Fontelis. In our trials, 1.5 pints per acre of Fontelis gives Sclerotinia control similar to one pint of Omega. Lower rates used for leaf spot and stem rot are not effective against Sclerotinia. Plan to follow up a Fontelis application in 2 to 3 weeks, or according to advisory. We have several trials this year examining different spray sequences for Omega and Fontelis and hope to have better information and options available in for future years.
Limit vine injury by limiting trips across the field, particularly when vines are heavy with water from rain or dew. Spraying a Sclerotinia fungicide at night when leaves are folded may get more fungicide directed to the lower canopy and also reduce vine injury. Keep in mind though that many Sclerotinia infections happen in areas above the crown. For example, spread by branch to branch contact, and even from leaves to branches is very common. Also, if Fontelis is being used, some reduction in leaf spot control is possible with night sprays. For these reasons, good coverage is needed for control, and drenching a Sclerotinia fungicide is not a good idea.
Unfortunately, all the Sclerotinia we are seeing this year is making all disease control decisions very tricky. Fontelis and Endura are group 7 fungicides and only two applications from this group should be made per year to limit the chances of fungicide resistance. Since Fontelis controls leaf spot, stem rot, and Sclerotinia, the risk of resistance development due to excess Fontelis use is especially important to consider. In Sclerotinia fields, avoid using chlorothalonil alone or mixed with other fungicides during August or in early September. Instead, use a group 11 fungicide like Abound or Headline, or a group 3 fungicide like Provost for leaf spot control. Once you are closing in on harvest (within 2 or 3 weeks), most of the yield loss from Sclerotinia will have already occurred and Bravo should be used for resistance management if at all possible. Fields without Sclerotinia should definitely be treated with Bravo for their final fungicide application.
In most years, mid-September brings dry weather and cool nights. In these years, leaf spot slows down so that sprays are not needed from early September until the crop is dug at the end of the month. In some years, warm nights and high humidity can continue into the fall. In those years, late-harvested peanuts may need continued fungicide protection. The best way to tell if you need to spray during this time is to follow leaf spot advisories, even if you use a calendar schedule the rest of the season.
As a final note, we have received many reports and have observed widespread incidence of leaf spotting. This spotting is not associated with a particular cultivar, although it seems to be most common in runner types. To date we have not identified a cause but are confident that these spots are not caused by early or late leaf spot fungi; in fact, we are fairly certain that no pathogen is involved at all. We are trying to identify the cause of this problem and will update County Agents as more information becomes available. Keep in mind that normal leaf spots rarely are visible before early September in sprayed, well rotated fields, and that the first spots will be very scattered across the field. In general, any symptom seen consistently and suddenly over most of the field is not due to a disease, but to some other problem like spray injury, drought, or nutrient problems.
Article first appeared as North Carolina Peanut Note (PNNC-2014-122)