The Disease Triangle and the Rotten Rainbow

The Disease Triangle and the Rotten Rainbow

Excessive Rain Causes Challenges for Fruit Growers

By Guy K. Ames, NCAT Horticulture Specialist

The so-called Disease Triangle is a Venn diagram that illustrates the interplay among the factors that cause disease. The factors that must be present for disease to occur are: a susceptible host, the presence of the pathogen, and a conducive environment. The photo at right shows a version I drew on my cider house wall. Below left is another version.

The Disease Triangle is a diagram showing how the intersection of a conducive environment, susceptible host, and pathogen results in disease.
Image: Wikimedia Commons

Look at that diagram and consider what happens if the environment is VERY conducive like it is this year in much of the eastern United States (i.e., imagine the yellow “Conducive Environment” circle larger). That’s right: you can have even more disease. We’ve had an overabundance of rain (“conducive environment”) and those susceptible fruit hosts (peaches, apples, strawberries, etc.) are succumbing to their respective pathogens like I’ve rarely seen.

Brown rot on peaches, black rot on apples, and gray mold on strawberries are rife this year. I’m calling this the brown, black, and gray “rotten rainbow.” Hey, I’ve got to try and squeeze a laugh out of this if I can! The black rot of apple is still in the leaf phase (called “frogeye leaf spot;” see Photo 1), but the conidia from the leaf infections will be infecting the fruit soon enough.

And strawberry growers—if they weren’t before—have become familiar with gray mold this year.  See Photo 2, courtesy of North Carolina State Extension.

And to round out our rotten rainbow, there’s brown rot of peaches from photos I took this morning (Photo 3).

I think we have a winner for the rottenest rot of the year (so far):  peach brown rot, you win!

Silver Lining in the Disease Triangle Cloud

Even with all this rotten news there is good news. IF you grow fruits like pawpaws, persimmons and pears, by and large, they are not susceptible hosts (refer to the Disease Triangle). And they are showing virtually no symptoms of rot-type diseases!

Also, if you want to focus on the “presence of the pathogen” part of the disease triangle, you could spray fungicides to eliminate the pathogen on the otherwise susceptible plants.

Finally, regarding the third factor of the disease triangle, the “conducive environment” (the climate), you might at first think that you couldn’t control or alter that. But you can create microclimates! One example is growing disease susceptible hosts in high tunnels. See the ATTRA publication High Tunnel Tree Fruits and Grape Production for Eastern Growers for more information. The other example is bagging individual fruit like I do with some percentage of my peaches (see photos below). Bags need to be put over fruit as soon as possible, when they’re about the size of a peanut.

Years like this one won’t convince me to stop trying to grow apples, strawberries, and peaches. However, I hedge my risks by growing a wide diversity of fruits and even individually bagging a few peaches. Next year’s weather could be different, but regardless, I will be eating some kind of fruit. Foster diversity!

More Information

For help identifying diseases on apples, apricots, cherries, peaches, pears, and plums, see ATTRA’s series of disease identification sheets in the Fruits section of the ATTRA Sustainable Agriculture website.

Guy and our other specialists are available to answer your questions about fruit diseases and many other agriculture topics! Give us a call at 1-800-346-9140 or email


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