One of the most important components of disease and nematode control in the cotton crop is “timeliness”. “Timeliness” means deploying the best management tactic to fight diseases and nematodes before these foes become well-established or before the opportunity to use a tactic has passed. August is a very important time in the cotton season to both be timely for possible fungicide applications this season and for preparation for next season.
As of the fist of August, there are four foliar diseases active in some, but certainly not all, cotton fields in Georgia. These include bacterial blight, caused by Xanthomonas citri pv. malvacearum, Stemphylium leaf spot, caused by the fungus Stemphylium solani, target spot, caused by the fungus Corynespora cassiicola, and areolate mildew, caused by the fungus Ramularia spp.
There is little that can be done to manage bacterial blight at this time in the season, other than to note if it has occurred in your fields and which varieties have been affected. Because of weather during the summer of 2018, even varieties which have shown some level of resistance to bacterial blight in the past have been affected this year.
However, now is the time that growers should note the level of bacterial blight in their crop and begin to make decisions as to variety selection for 2019. Growers have an increasing number of “bacterial blight resistant” varieties from which to choose.
Stemphylium leaf spot is characterized by numerous small spots with dark purple/brown margins and often times gray, papery centers. This disease occurs when the cotton plant is deficient in potassium; potassium deficiencies may exist because of poor soil fertility, perhaps from leaching, or during periods of drought where potassium is not taken up into the plant. Stempylium leaf spot is managed by insuring proper levels of potassium in the plant; fungicides are not an effective management tool.
Target spot became evident in southwestern Georgia in the latter part of July and is likely present in many fields across the Coastal Plain of the state. Target spot can develop quickly and is most common in good-growing cotton with high yield potential. Extended periods of leaf wetness, where the foliage in the interior of the canopy remains wet well into the later morning hours, create perfect conditions for rapid development of target spot and premature defoliation from it.
Fungicides are an important management tool for target spot, though use does not always result in increased yields. From our research, effective use of fungicides should be considered between the first and sixth week of bloom where the third week of bloom is typically the most critical time of management.
Scouting before use of fungicides to determine if the disease is present help to ensure that an application is warranted. Priaxor is currently the most effective fungicide for control of target spot, though Headline, Quadris and others are also effective.
Areolate mildew was especially severe in 2017 and there are reports that the disease is back in 2018 if a few fields. Areolate mildew has historically been confined to southeastern Georgia east of I-75; however it can be found elsewhere as well. Typically arriving too late in the season to cause any damage (in fact, late-season defoliation may be a benefit), use of fungicides had often not been warranted.
However, in severe cases, the same fungicides used to control target spot are also effective in the management of areolate mildew. Growers within three weeks of defoliating their cotton need not worry about managing areolate mildew. Where areolate mildew occurs in a crop with anticipated defoliation a month or more away, and weather is favorable for continued development and spread of the disease, then use of a fungicide may be beneficial to protect yield.
Unfortunately, we have very little data at this point with which to refine our recommendations for management of areolate mildew.
Nematodes and Fusarium Wilt
It is too late to protect our 2018 cotton crop from plant-parasitic nematodes of Fusarium wilt; however now is the time that symptoms become very evident in the field. Where stunting, poor growth and even dying plants are found in areas of a field, growers should take measures to determine
- is it caused by nematodes?,
- if so, what kind of nematodes, and
- is Fusarium wilt also involved.
Detection and identification now will help growers to make best variety selection and possible use of nematicides in 2019.