Not all Boll Rots are caused by Bacterial Blight

Cotton boll rots can be a confusing topic to discuss. A laundry list of microorganisms including bacteria, fungi, as well as yeasts comprise the specific complex of organisms that cause boll rot.  In addition, insects can produce a wound on bolls that in some cases can allow microorganisms into the developing boll and result in boll rot or a general staining of the cotton contained within the boll.  However, not all insects are capable of wounding bolls and unless documented as a problem in the field should not be implicated in causing boll rot.  Nor are insects the cause of the large increase in bacterial blight that has been observed in the Mississippi cotton production system over the past three years.  Following a year like 2014, with what could be considered below normal temperatures, but above normal precipitation, we have encountered more boll rot this year than in the past two years.  Some of this can be blamed on bacterial blight, especially in situations where bacterial blight was observed on bolls prior to defoliation; however, caution should be taken when blaming all boll rot on bacterial blight simply because the disease was so widespread during 2014.  Keep in mind, bacterial blight will only be observed on susceptible varieties and therefore will not have resulted in boll rot on varieties with documented resistance/tolerance to the bacterium.

Generally speaking, boll rots occur deep in the cotton canopy where ample moisture is present. In any given year, rotted bolls, as well as bolls with hardlock where the boll will remain unopened even at maturity, can be observed in low, scattered quantities in almost every cotton field.  One thing is certain, bacterial blight, when present on bolls, can be one of the worst boll rotting organisms.  However, not all boll rots are initiated by bacterial blight and determining the initial boll rot culprit in a field that contained bacterial blight is even more difficult.  Late in the season the characteristic water-soaked lesions present on bolls can be obscured by secondary organisms or not present at all even if the disease was present on leaf tissue.

Once the leaves have been removed, bolls have begun to naturally open and a boll opener has been applied to a cotton field it can be extremely difficult to determine the presence of bacterial blight. Bacterial blight leaf symptoms are no longer present, petioles are generally chemically burned and the characteristic “cigarette burn” symptoms on bolls have been obscured, in most cases, by other organisms (see included photos).

Boll rot as caused by bacterial blight beginning at the base of the boll and indicated by the water-soaked lesion’s appearance.

Over the past two months we have received numerous telephone calls regarding the presence of boll rot, especially in fields of bacterial blight susceptible cotton varieties. In some of those cases, bacterial blight had been observed in the field earlier in the season.  The organism that causes bacterial blight is in fact one of the worst boll rotters in a list of organisms, that depending on the source, can include up to 200 different microorganisms.  However, in situations where bolls were infected by the bacterium, secondary organisms can easily infest the boll lesions and obscure the presence of bacterial blight (see images).  The entrance of the bacterial blight organism into a developing boll through natural openings and the water-soaked lesion that results as a symptom of infection are an easy area for secondary organisms to invade and continue to rot the boll.  The release of sugars and additional nutrients from the boll that can occur as a result of bacterial blight can attract secondary organisms and make distinguishing the initial symptoms of bacterial blight extremely difficult.

Cotton fields should be scouted for the presence of bacterial blight long before harvest preparation. Two weeks ago one field in particular was scouted in Bolivar County.  Unopened bolls in the upper canopy that remained unopened following the defoliant and boll opener application contained bacterial blight symtpoms.  However, the majority of the symptoms present on bolls that were already opened and bolls that appeared to be rotted could be identified as secondary fungal infection (see attached images with pink fungal growth present on bolls).

Secondary boll rot, likely caused by Fusarium as developing within an initial water-soaked spot related to bacterial blight.

Boll rot as a result of Fusarium (as indicated by the pink growth of fungal structures on the boll).

Boll rot/hardlock as a result of a fungal organism.

For more information regarding bacterial blight in Mississippi during 2014 see:

https://www.mississippi-crops.com/2014/07/18/cotton-bacterial-blight-update-july-18-2014/

For more specific information regarding boll rots see:

https://www.cotton.org/tech/physiology/cpt/pest/upload/CPT-Sep94-REPOP.pdf

http://www.cottoncrc.org.au/industry/Tools/Symptoms_Identification_Tool/Cotton_Symptoms/Boll_rot