Tropical storms can pose a triple threat to disease control: conditions that are highly favorable for disease, potential loss of fungicide activity, and fields too wet to spray. Leaf spots, particularly late leaf spot, Rhizoctonia limb rot, web blotch, and Botrytis can really take off following heavy fall rains. Wet fields and cool nights make a nightmare scenario for Sclerotinia blight.
Leaf spots, stem rot, web blotch
Under normal conditions, we expect foliar fungicide protection to last 10 -14 days, but heavy rains can lead to some wash-off and shorten the protection period. Headline probably is our longest-lasting foliar fungicide and will give excellent protection against late leaf spot and web blotch at 9 -12 oz/a. If you feel you need additional protection against stem rot, consider adding 13-16 oz Convoy to a Headline spray since Convoy is also long-lasting. Keep in mind that Convoy has a PHI of 40 days, however. Provost at 8-10 oz/a is active against leaf spots and stem rot and can provide the needed protection. Abound at 12-18 oz/acre would be a good choice for fields at risk for Rhizoctonia limb rot based on history and/or heavy vine growth. Avoid using tebuconazole since it is very weak against late leaf spot and vulnerable to wash-off.
We underwent a period favorable for Sclerotinia about a week ago and sprays were advised in many locations. Omega lasts for at least 3 weeks, so fields sprayed recently should be protected for now. Scout carefully. If Sclerotinia is active in fields with a strong history of disease, and you have not sprayed recently, consider making an application in anticipation of Irene, again remembering that the PHI for Omega is 30 days.
Barbara Shew | Research Assistant Professor
Director, Plant Disease and Insect Clinic
Dept. of Plant Pathology | NC State University
100 Derieux Place | 3404 Gardner Hall
Campus box 7616 | Raleigh, NC 27695-7616
919.515.6984 (O) | 919.810.8735 (M)
Clinic web page http://www.ncsu.edu/pdic
My web page http://www.cals.ncsu.edu/plantpath/people/faculty/shew-b/