I thought I’d take a little time this month to do an update on a potato problem that we all have to deal with at one time or another: Fusarium dry rot.
This storage and seed decay disease has probably been with us as long as we have been cultivating potatoes. There have been, however, some developments in the last several years that make me think a short update is necessary.
The seed decay aspects of this disease complex certainly are important, but I will be concentrating on the storage rot phase for this discussion.
Fusarium dry rot in potato in North America is usually associated with two causal fungi: Fusarium coeruleum and Fusarium sambucinum. These two species are distinguished from one another by the pattern of infection in the potato tuber.
F. coeruleum rots the tuber in a very regular pattern with the decayed tuber flesh a yellow to pale brown in color. In contrast, F. sambucinum causes a very dark chocolate brown to black rot that attacks the tuber in an almost random, non-uniform manner, often leading to a “tunneling” effect.
F. coeruleum seems to be more important as a seed decay agent and F. sambucinum as a storage pathogen, but they can readily reverse these roles.
To this list we can also add Fusarium graminearum, the same fungus that is responsible for Fusarium head blight in cereals. Although this fungus appears to be quite common, causing storage rots in European potato production, its role in North American production is a relatively new discovery.
The tuber symptoms produced by F. graminearum are identical to those produced by F. sambucinum. This similarity may be one reason why we have only recently realized that F. graminearum is causing storage rots on this side of the Atlantic.
It is certainly no secret that most of the isolates of F. sambucinum are resistant to thiabendazole (Mertect).
High on my “wish list” for the new year is the availability of some new postharvest fungicides that will be effective against Fusarium dry rot. We really have a very short list of materials that can be applied postharvest and some of them, like phosphorus acid, aren’t effective at all against Fusarium. There are also some biological postharvest treatments available that can do a pretty good job, if the disease pressure from dry rot isn’t too intense.
On the good news side, all of the F. graminearum isolates that have been characterized so far and all but a few isolates of F. coeruleum are not resistant to thiabendazole.
Remember that one of the best ways to minimize dry rot is to avoid damaging potatoes at harvest.
All of the dry rot fungi require a wound before they can infect potatoes—these fungi cannot infect an intact potato tuber skin.
Allow enough time for adequate skin set before harvesting. Also highly recommended is avoidance of harvest when pulp temperatures are high (over 65 degrees F) since excessive heat always makes this disease worse.
There are new varieties coming into more widespread production that appear to be more susceptible to dry rot than some of our traditional varieties. Some of the same variety features that make them so attractive to consumers—like thin, delicate skins—also make them more susceptible to dry rot, especially if they are mishandled.
Extra care when harvesting potatoes is never bad advice, and if you are working with these newer varieties, paying extra attention to skin set, bruise management and proper pulp temperatures during harvest is highly recommended.