Late blight is a very destructive disease of potato and tomato. This disease first became known worldwide 150 years ago as the cause of the “Potato Famine of Ireland and Northern Europe.” It has gained new notoriety a few years ago as new strains have begun to appear in North America and Europe that are more aggressive and resistant to previous effective fungicides.

Luckily, late blight has not been much of a concern in the Southern San Joaquin Valley the past three years due to the dry, warm spring weather we have had. Often we forget about important things with the mentality of out of sight, out of mind. But we only need to go back a few years to 1998. During the El Nino year late blight was a major problem in potatoes and tomatoes.

It would be prudent for growers, managers, and consultants not to get too complacent with late blight based on the history of just the past few years. As we enter the rainy season in Southern California and potatoes are being planted, with tomatoes soon to follow, it could mean that late blight could also become a problem again.

Late blight can develop and spread rapidly under ideal conditions and when spores of the fungus are present. Therefore growers, managers, and consultants need to keep a vigilant eye out for this problem, even though it has not been an issue the past few years. Ideal conditions would be high humidity or free moisture with temperatures 50 degrees to 78 degrees. But late blight can certainly be active at temperatures above 78 degrees also.

Preventive measures

Preventive control measures should be used to help avoid late blight altogether. These include using certified seed whenever possible, destroying volunteer potato and tomato plants, avoid making cull piles, scout fields as often as possible, and the use of preventive applications of fungicides when weather conditions favor the disease.

Although late blight has not been a major issue in California the past few years, it is still advisable to be on guard for this problem. Late blight leaves little room for a second chance at the rate that it can move across a field when conditions are ideal.