Rhizopus rot or “leak”

/ Gerald Holmes
Figure 1. Rhizopus rot is diagnosed by the conspicuous sporulation on the fruit surface. Under high humidity, sporulation appears “hairy” but is more appressed under drier conditions. Rot progresses very quickly and fruit become liquified within a couple of days. (photo by G. Holmes)

June is here and temperatures are rising. Along with rising temperatures comes a shift in fruit rotting fungi from Botrytis to Rhizopus (Fig. 1). Rhizopus stolonifer causes Rhizopus rot or “leak”. The term “leak” comes from how the fruit are liquified by the fungus and “leak” out. Whenever you see a wet trail running down the side of the plastic mulch, chances are good there’s a Rhizopus rotted fruit in there somewhere (Fig. 2).

Rhizopus rot can also be a problem postharvest but this usually indicates that fruit were stored above the optimum temperature of 34F (Fig. 3). It’s critical during periods of high temperature that fruit are harvested in the cooler hours of the day and be transported to the cooler within two hours of harvest. Pallets of clamshells can heat up fast if they are left in direct sunlight for even a few minutes. Find or create shade for the period between harvest and transport to the cooler. Rhizopus can be controlled by rapid cooling and storage at 34F.

Figure 2. Wet trail running down the side of the plastic mulch indicates Rhizopus soft rot. This is particularly conspicuous on white plastic (left). Often the rotted fruit is hiding beneath the plant canopy, but the wet trail gives it away (right). (photos by K. Blauer [left] and G. Holmes [right])

Few fungicides have any activity against this disease. Switch and Miravis Prime both contain fludioxonil and this is one of the few fungicides active against Rhizopus rot. Merivon is another fungicide we’ve seen good efficacy from (Fig. 4.). Keep this in mind as temperatures rise in the coming weeks and you see more Rhizopus rot.

Figure 3. Rhizopus rot can occur postharvest, especially if proper temperatures are not maintained. This fungus grows very fast and will spread to fruit throughout the clamshell very quickly. Prompt cooling to 34F is the best way to control this disease postharvest. (photo by G. Holmes)
Fungicide efficacy table( PDF)
Figure 4. Click on the image or here to access the fungicide efficacy table (PDF).