Features Agronomy Diseases
Heat causing two potato problems

Heat edema has been observed in Ontario, and should disappear when humidex values drop below 30 C. Photo courtesy of Eugenia Banks.

July 27, 2016, Ontario – Two potato problems have developed due to high temperatures, writes Eugenia Banks in her latest Ontario potato update.

Fusarium wilt
The long and relentless heat wave that is affecting Ontario has provided favorable conditions for diseases and physiological problems that we do not often see. One of them is Fusarium wilt caused by Fusarium oxysporum, a relatively common soil fungus that thrives when soil temperatures are around 29 C. 

A grower found patches of Fusarium wilt this year in a field of Andovers. Also wilted plants here and there were noticeable in the field. The base of the stems emerging from the seed was woody. At ground level the stems were dark brown, rotten or hollowed but did not have a strong fishy smell. The upper part of the stems was still green but wilted. Most of the plants pulled had rotten seed, but with no fishy smell.

Gary Secor from North Dakota State University helped with the identification of this problem. He isolated Fusarium oxysporum from the diseased stems. There was no sign of blackleg, dickeya or verticillium. 

How to control this disease? 
Fusarium oxysporum is more prevalent in very hot summers, just like our 2016 growing season. There is nothing that can be done about the weather, but: 

● Avoid ammonium nitrogen as a source of N 

● Keep potassium levels adequate 

● Acid soils favour Fusarium wilt. Liming soils help, but may increase the risk of common scab. 

Heat edema
This problem has been observed in two fields of the variety Canela Russet, growing in two different production areas. Small bumps form on the leaves that, with time, rupture leaving round, brown necrotic spots. The centres of the spots often drop out leaving holes in the leaves. Holes in leaves usually means insect feeding, but there were no insects in the fields. Ian MacRae, an entomologist at the University of Minnesota, did not think that the holes looked like insect feeding. Jeff Miller from Miller Research in Idaho suggested heat edema, a physiological disorder. Edema is related to water retention (see page 122 in the Potato Field Guide), but heat edema is different. The cells expand trying to diffuse the heat and eventually rupture. Because it is a physiological problem, heat edema should disappear when humidex values (temperature plus humidity) drop below 30 C. Yesterday, I checked one of the fields and noticed that, because of the hot weather, necrotic spots were developing on new leaves. 

Prolonged heat waves bring all sorts of different problems to potato plants. Working with potatoes is a never-ending learning experience! 

July 27, 2016  By Eugenia Banks