Prior to the recent dry weather this past week, the generally wet and cloudy weather patterns over much of August across Alabama were favorable for the development of foliar diseases in cotton and peanut. The disease risks in both crops was enhanced by the frequent showers and wet soils which interfered with routine protective fungicide applications.
In peanut, early leaf spot has appeared in Georgia-09B and Georgia-16HO at the Wiregrass Research and Extension Center. In addition, late leaf spot was found at the Gulf Coast Research and Extension Center and production fields in Georgia-06G in Central AL. If showers continue into September, expect to see both leaf spot diseases getting a foothold in peanuts across the state, particularly in those fields where fungicide applications were delayed by wet weather.
Early peanuts that will mature in the next two or three weeks probably will have few leaf spot issues. Typically, later maturing peanuts are more at risk from early and late leaf spot control failures due to higher inoculum levels encountered in September and October.
Control of ongoing leaf spot epidemics is exceptionally difficult. When leaf spot incited defoliation is begun, producers are advised to 1) shorten application intervals from 14 to 10 days, 2) use aerial application to stay on schedule, 3) tank mix 0.75 to 1 pt/A chlorothalonil (Bravo, Echo, Equus, etc.), methyl thiophanate, tebuconazole, or other generic leaf spot fungicides, with their premium leaf spot/white mold fungicide, or 4) apply 6 to 8 fl oz/A Priaxor or 3.4 fl oz Miravis alone or in combination with Alto @ 5.5 fl oz/A or 7.3 or 9.5 oz Elatus. In 2017 Alabama field trials, the best leaf spot control was obtained with Miravis when tank mixed with Elatus. In the past, pyraclostrobin (Headline) at 9 fl oz/A also provided good leaf spot protection. However, tolerance to strobilurin fungicides is an issue in some peanut-production areas, so this fungicide along with azoxystrobin may no longer provide effective leaf spot control. Generic formulations of azoxystrobin alone or in combination with a triazole fungicide (i.e. flutriafol, tebuconazole, or tetraconazole) or chlorothalonil as well as pyraclostrobin alone are available.
If leaf spot control is good, producers need to continue protective fungicide applications need to continue up to 14 days before harvest, particularly in late maturing peanuts and always watch for a late summer tropical storm.
One disease that has not yet popped up is peanut rust, which typically shows up first in Baldwin or Mobile Co, particularly in later maturing peanuts. Last fall, this disease hit a late planted peanut cultivar trial in DeKalb Co! Once established, this disease is much more difficult to control than leaf spot diseases. Rust 'hot' spots look just like spider mite damage but appear well away from field borders or light poles. Producers in southwest AL need to start scouting for rust 'hot' spots. If rust does appear, apply 1.5 pt/A chlorothalonil at 7 to 10 day intervals. The efficacy of premium leaf spot/white mold fungicides under severe rust pressure is largely unknown. To my knowledge, we've not had many opportunities to screen many of the new fungicides for peanut rust control.
The past year or two, foliar diseases have not been a major issue in Alabama cotton. They are now. With the wet weather patterns over the past few weeks coupled with rank cotton, target spot is a real threat to significantly reduce lint yield in some Alabama cotton fields. Damaging outbreaks of this disease have been reported across Georgia. Yield losses to this disease on a susceptible cultivar can hit 400 lb lint per acre. That translates into a $320 to $360 per acre income hit at this year's market price. Fortunately, the majority of Alabama cotton acreage is cropped to less susceptible to partially resistant cultivars (primarily Deltapine 1646 B2XF among others). Recently, fields of this cultivar in Butler and Escambia area were almost target spot free. However, OVT full season flex cotton cultivar trials at WGREC and Prattville along with trials at Brewton, Prattville, and Tallassee, which include many 'partially' resistant cultivars, are getting plastered by target spot. Defoliation levels in the range of 75% along with sizable yield losses are anticipated by September 1. The cotton in all of the above trials has rank growth; in contrast, there's little disease activity in trials (i.e. GCREC and Field Crops) where the cotton has not lapped the middles. Rank top growth is a must for severe target spot outbreaks and significant yield loss.
In addition, areolate mildew (false mildew) has made an early appearance in Alabama cotton and probably more of an issue than target spot. In the past, significant outbreaks of this disease have been few and far between but this disease caused concern last year in Georgia. Little is known of the impact of weather on disease development or the susceptibility of cotton cultivars to this disease. Disease outbreaks were observed over several years in a cotton-sweet sorghum rotation study at Brewton (BARU). There, disease intensity was higher in continuous cotton than cotton cropped behind sorghum. Also, differences in the reaction of Phytogen 499WRF and Deltapine 1252 B2RF were noted. Prior to this year, areolate mildew has not been a serious issue in any cotton fungicide screening studies at Brewton, Field Crops, or GCREC.
Areolate mildew outbreaks have primarily been reported this year in West Central and Central Alabama. Severe defoliation has been observed in some fields, particularly in West Central AL, where a sizable percentage of the cotton acreage has been treated with a fungicide. This disease has developed in some of the OVT cultivar trials and other cotton cultivar trials at other South and Central AL locations but disease intensity has been low. No obvious differences in the reaction of cotton cultivars to areolate leaf spot have been observed.
Unlike target spot, the relationship between areolate mildew-incited defoliation and yield has not been established. However, the defoliation pattern for early onset areolate mildew appears to be similar to target spot, so I would anticipate the yield loss potential when defoliation levels exceed 50% are probably similar for these two diseases.
At this point, early May cotton has reached the 7th week of bloom and will no longer benefit from fungicide protection. In Alabama, trials have shown that a pair of late applications of Headline or Priaxor (5th and 7th week of bloom) can give excellent yield protection and good control of target spot. If wet weather patterns return, there may be issues with this disease in late May and June-planted cotton. Unfortunately, there's no information on the performance of single late fungicide application programs against either disease. In a Georgia trial some years back, Headline gave good control areolate mildew. Information on the efficacy of other fungicides against areolate mildew is a bit thin. I expect that Priaxor, which contains pyraclostrobin, will also have good activity against this disease as would azoxystrobin (Quadris, Topguard EQ, and generic products), and Elatus (azoxystrobin + benzovindiflupyr). In some Deltapine 1646 B2XF in Butler Co., a single application of azoxystrobin appeared to have given good control of areolate mildew. Areolate mildew has not hit any of the fungicide screening trials at Brewton, Field Crop Unit, or Gulf Coast, so fungicide efficacy data for this disease will not be generated at those locations this year.
One saving grace is the onset of drier weather patterns over the next 10 days across much of the state. Those conditions should slow if not stop target spot and areolate mildew activity in cotton and will help suppress leaf spot diseases in peanut.
Agronomic Crops, Professor
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