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Which fungus are you trying to control?

Is it early blight? Black dot? Brown spot? Not surprisingly, growers wonder if their choice of fungicide is the right one. The sad fact is there are no fungicides registered in Canada labelled for control of the latter two pathogens. There are fungicides registered for early blight, but if a grower does not have early blight and one of the other two instead, is it a waste of time and money spraying?

Typical late blight leaf lesions.

Identification is key, according to Dr. Tracy Shinners-Carnelley, a potato pest specialist with Manitoba Agriculture, Food and Rural Initiatives. However, she adds, it is difficult to separate the three in the field because they are so similar and, in fact, two or all three could be present at the same time.

“I did a little exercise for a meeting in 2007 and fooled a lot of people, including myself!” Shinners-Carnelley says. Prior to the meeting, she collected early blight and brown spot-looking lesions. Then, she took the samples back to the laboratory for a definitive identification. Attendees at the meeting were shown photos of the lesions and quizzed on the pathogens responsible. “We all discovered there was no easy identification of the lesions.” She adds that researchers are now beginning to refer to the trio as “leaf lesion complex” because, without a test in a laboratory, identification is difficult.

“In the end, it may not matter which is which because the present fungicide options are not specific for the individual diseases,” Shinners-Carnelley admits. “Early blight is the predominant disease in the complex, so fungicide selection is targeted at this pathogen. If there truly were three products available for each lesion, identification would be more critical.”

Where does that leave growers who believe they have a problem but do not know which one? Shinners-Carnelley says an integrated approach to disease management is important. Where possible, production practices should reduce crop stress since a healthy crop is less susceptible to the leaf lesion complex, particularly if the environmental conditions do not encourage pathogen growth. “Growers shouldn’t rely only on fungicides to protect their crop,” she continues. “You still have to grow a healthy crop.”

Shinners-Carnelley says there is some work being done in the US on identification, control and economic impact of leaf lesion complex. She says research suggests that some of these
diseases may not have a serious impact on their own, but combined with one of the others the losses could escalate. She adds that one lesion will attach to another and that is when the problem can grow.

Early blight lesions have ‘target board’ growth rings and weakened plants show yellowing of leaves.

There are fungicides registered for early blight in the US that advertise brown spot and black dot control but the Canadian label does not make that claim. Meanwhile, there is no research completed in Canada to indicate how much damage brown spot and black dot cause on their own. What is known is that combined with early blight, growers have a bigger problem.
The solution, therefore, is to do everything possible to ensure a healthy crop. Use a registered product to prevent early blight and, if other leaf spotting pathogens are present in the field, there may be peripheral control. Canadian growers would be wise to use products conscientiously and to be sure they are correctly targeted. Shinners-Carnelley says despite there being resistance to some early blight fungicides in the US, growers are still using them because they control black dot. This practice could lead to more serious resistance in the future.

Leaf lesion complex is a complicated issue. Until there is more research on the three pathogens, control recommendations are targeted toward the entire complex rather than on the individual diseases. Growers need to rely on their own or a crop advisor’s assessment of the problem and then take appropriate action. In addition to fungicide use, maintain crop health to ensure leaf lesion complex pressure is minimized when the crop is in its important growing stage. -end-