What is in this article?:
• While target spot is found in dryland cotton, heaviest leaf spotting and defoliation has been seen in irrigated cotton, particularly when strip- or conservation-tilled.
• In 2012, disease outbreaks were observed in cotton in the Florida Panhandle, South Carolina, North Carolina and Virginia.
TARGET SPOT IN cotton continues to spread throughout the Southeast and has caused yield loss in some fields.
Target spot is by no means a new disease of cotton, but it’s certainly attracting the attention of more producers as it spreads throughout the Southeast, says Austin Hagan, Auburn University Extension plant pathologist, speaking at the recent Central Alabama Row Crops Workshop in Autaugaville.
Target spot, which is caused by the fungus Corynespora cassiicola, was first noted on irrigated cotton in southwest Georgia in 2003, says Hagan, and lint losses due to the fungus have been estimated at 200 pounds per acre.
“In some instances, defoliation may occur so late in the production cycle that the disease served more of a harvest aid and had minimal impact on yield. But starting in 2011, target spot outbreaks were noted statewide in Alabama cotton,” he says.
While target spot is found in dryland cotton, heaviest leaf spotting and defoliation has been seen in irrigated cotton, particularly when strip- or conservation-tilled, notes Hagan.
In 2012, disease outbreaks were observed in cotton in the Florida Panhandle, South Carolina, North Carolina and Virginia.
The disease was observed beginning in 1959 on cotton in Mississippi and may have been present in the Mississippi Delta for 25 years prior to that observation, says Hagan.
“While the mechanism for the rapid spread of target spot in cotton has not been determined, the fungus is transmitted through seed of soybean and sesame. While seed transmission may account for the rapid spread of target spot into Alabama and up the eastern seaboard, the fungus may have moved from soybean or vegetables into cotton.
The spotting, says Hagan, follows a different pattern than that seen in potash-related leafspot.
“We do have issues with potash-related leafspot in cotton, but those are not target spot. There are several fungi associated with these leafspots, but they’re more associated with a potash deficiency or a combination of drought, rapid boll set, or a lack of uptake of potash or other minerals by the plant.”
Potash-related leafspot is mostly an issue on high-yielding, early maturing varieties, particularly on sandy soils where the minerals tend to move quickly through the soil profile, says Hagan.
“It seems to be triggered by moisture stress and rapid boll set. If the plants aren’t fruiting heavily, they don’t show the symptoms. But when they set a lot of bolls, you start to see the yellowing in the canopy and the spotting, and then the leaves fall off.”
There are fungicides registered for the control of these types of leafspot, he says, but research from the University of Georgia has never shown significant yield response. But when potash is applied earlier in the season, a positive response is seen.
With target spot, says Hagan, the plant starts shedding the older leaves and it moves upward through the canopy.
“It follows a classic leafspot pattern like we see in peanuts, starting at the bottom and moving up, towards the shoot tips. These spots are large, discreet and usually circular. The color can vary, but usually they have a tan center and a darker outer ring.
“If you look closer, you see a concentric ring that’s consistent with this leafspot disease. Occasionally, if it’s really wet outside, you might see some type of leaf blighting.”
In the last seven years, says Hagan, the epicenter of the target spot problem has been in southwest Georgia on irrigated cotton.
“It was probably in Alabama before 2011, we just didn’t look very hard prior to that time. We’re not sure why it has moved so quickly over such a short period of time. The type and size of this fungus won’t move in thunderstorms or in air currents over long distances. It also goes to tomatoes and cucumbers, so it could have moved from vegetable crops.”