Protecting cotton yield from foliar diseases requires a multi-faceted approach that includes variety selection, rotation, fertility management, tillage practices, and avoiding rank growth.
A little help from Mother Nature doesn’t hurt, either.
Variety selection is a good first step in managing bacterial blight in cotton, but may not be as effective for target spot, a relative newcomer to Delta cotton with potential for yield losses as high as 150 pounds of lint per acre. No variety has a high level of target spot resistance, and variety reaction to the foliar disease differs widely, according to a University of Tennessee Institute of Agriculture Extension plant pathologist.
Dr. Heather Kelly says either disease requires three main factors to develop, these are the factors under the disease pyramid — environment, pathogen and plant. Bacterial blight develops in wet conditions from rain or irrigation. Target spot likes wet/humid and hot. Both diseases have been observed as early as July in Tennessee.
While both diseases cause foliar symptoms, the entire field and all symptoms should be considered when diagnosing these diseases, Kelly warned. “In 2017, bacterial blight was easily mistaken for target spot when only focusing on lesions that had concentric rings; but considering the yellow halo around these angular lesions (image 1) and that the lesions were in mid to upper canopies and edges of rows (image 2), and other symptoms, including water soaking of lesions (image 3) and brown, necrotic leaf veins (image 4) could be used to identify bacterial blight as the culprit.
Later in the season, water soaked lesions may appear on bolls (image 5) and eventually cause boll rot. True target spot lesions (image 6) begin in the lower canopy, after canopy closure. Lesions only progress to the middle and sometimes upper canopies when significant defoliation of the lower canopy has occurred (image 7).”
Kelly, speaking at the recent Cotton Focus Conference in Jackson, Tenn., said the earlier the disease develops, the greater the chance of affecting yield. Boll rot is the main yield loss mechanism of bacterial blight, while target spot causes defoliation that has been correlated to yield loss, she said.
Target spot typically affects the lower canopy after the canopy closes. Factors that may increase the risk of target spot include:
- No- or strip-till cotton fields of cotton followed by cotton.
- Frequent showers and/or irrigation.
- High nitrogen fertility levels.
- High-yielding varieties are most affected.
- Rank growth.
- Field history.
More to Learn
She said research scientists and Extension specialists are learning more about the disease. “Here’s what we know: Fungus survives on cotton debris; infection depends on variety and location; yield loss can be 150 pounds of lint per acre or more; higher losses are more likely as yield increases; target spot develops after canopy closure; and late planed cotton is at lower risk.
She said research has more to learn, including: the yield loss mechanism, range of yield loss for most varieties, crop rotation effect, influence of tillage practices; irrigation; and geographic location.
Key management strategy for target spot includes scouting, starting around first flower and canopy closure; examining risk factors; and considering fungicide applications on a field-by-field basis.
Fungicides are not a guarantee of improved yield, she said. “Based on regional data from 2014 — 2016, only 20 percent of the time did a fungicide significantly protect yield. That probability will decrease with incorporation of 2017 data. Based on Tennessee trial data from 2014 to 2017, only 15 percent of the time did a fungicide significantly protect yield. On average, fungicides protected around 174 pounds of lint per acre, for that 15 percent when a fungicide significantly protected yield.
She said later fungicide applications had best probability of preventing defoliation and protecting yields.
Variety selection is the first option to protect against bacterial blight. Producers have a good selection of resistant varieties, including:
- Phytogen330 W3RF
- Phytogen340 W3RF
- Phytogen430 W3FE
- Phytogen440 W3FE
- Phytogen480 W3FE
- Stoneville 5517 GLTP
- DeltaPine1518 B2XF
- DeltaPine1820 B3XF
A few others are considered mostly susceptible but partially resistant, including:
- DeltaPine1614 B2XF
- Phytogen312 WRF
- Phytogen444 WRF
- DeltaPine1646 B2XF
These are susceptible:
- Stoneville 4949 GLT
- DeltaPine1522 B2XF
- DeltaPine1725 B2XF
- NexGen3406 B2XF
- NexGen3522 B2XF
Producers should consider crop rotation and tillage management to minimize bacterial blight.
“Bacterial blight has had a minimal impact on yield so far in Tennessee,” Kelly said, “with little boll rot being observed so far.”