2011 Cotton Bacterial Blight Trial Ratings: Stoneville, MS

By: Tom Allen, Bobby Golden, Gabe Sciumbato, Darrin Dodds, and Peggy Thaxton

Two additional OVT trial blocks were planted in Stoneville, MS (2 row, 40 foot plots) this season.  Several weeks ago one of the blocks was inoculated with the bacterium that causes bacterial blight (Xanthomonas axonopodis pv. malvacearum race 18) the other block remains noninoculated.  Little if any data exists to suggest the yield reduction that could be expected from a light, moderate, or heavy bacterial blight infection or if there are differences regarding when the infection occurred.  Generally speaking the only available yield loss guideline is the one included in the Compendium of Cotton Diseases that suggests yield loss associated with the disease can range from 5 to 30% lint yield loss.  However, most of the work that considered yield loss was conducted in older varieties several decades ago.  Historically speaking, in MS, in 1972 a bacterial blight epidemic occurred.  Dr. Billy Moore suggests that a 10% reduction in overall yield occurred that year across the entire state.

On Monday (July 25, 2011), the inoculated block was rated to determine if infection occurred post-inoculation (the noninoculated block was observed for the presence of disease which at this time was absent from the plots).  The rating information is included to provide a guideline on the susceptibility on cotton varieties planted in MS.  Five of the varieties included in the OVT trials were considered to be resistant to bacterial blight while the other 17 had symptoms of the disease develop on leaf material as well as leaf shed.  Plots were rated by observing three separate locations within each plot on a scale of 0-9 using the general scale outlined below.

Rating Scale (0-9):

0 = no disease or defoliation from bacterial blight

1 = bacterial blight present (i.e. single lesion counted as 1)

2 = infected material present in lower canopy

3 = mid-canopy infection and some defoliation

4 = heavy mid-canopy infection and some defoliation

5 = mid to upper-canopy infection and some defoliation

6 = upper canopy infection and defoliation

7 = heavy upper canopy infection and defoliation

8 = majority of upper canopy infected with excessive defoliation

9 = total defoliation of plant