Aug 3, 2020
Preventing apple fruit rots; pruning blossom-basted stone fruit trees

The rainfall in Pennsylvania due to summer thunderstorms has been feast or famine, depending on where you are located. The hot, dry weather has decreased disease pressure; however, we are not out of the woods.

As we near apple harvest, it is still important to be vigilant when managing for fruit rots, especially those fruit destined for long term storage. There is another benefit to the hot, dry weather we have been experiencing: management of the disease headaches caused by Pseudomonas syringae pv. syringae this spring.

Here are some nuggets of disease management wisdom as we enter August:

Preventing pre- and postharvest apple fruit rots

Be mindful of any late-season shoulder cracking on apple fruit since such wounds can be an easy opening for fungal spores to cause fruit rots. Since bitter rot has been the primary concern in the last few years, our research to date has shown conventional fungicides vary in efficacy, even within a fungicide mode of action group. Our data support Merivon (FRAC group 7 + 11; 0-day PHI) to be the most effective late-season control.

Merivon will also afford rot protection during storage, as well. The predominant fungal species causing bitter rot in our region is not susceptible to trifloxystrobin, which is found in Luna Sensation and Flint Extra; however, the species is susceptible to pyraclostrobin, which is found in Merivon. If bitter rot is not a concern, any of the fungicides will be effective, in addition to thiophanate-methyl (FRAC Group 1; Topsin, T-methyl, etc.)

Some general management techniques to keep in mind to reduce postharvest fruit rots:

Bruised or wounded fruit are susceptible to postharvest fruit rots, such as blue mold and gray mold. While harvesting, handle fruit carefully when picking and transferring fruit from bag to bin to avoid bruising or wounding.

  • The more mature a fruit, the more susceptible it is to storage diseases. Harvest fruit at proper maturity.
  • Inoculum sources for rot pathogens causing disease in storage (if already not hitching a ride on the fruit) come from plant and soil debris. Use clean bins and minimize the amount of soil and plant debris brought in on bins.
  • Warm temperatures encourage pathogens to grow. Keep fruit cool after harvest, i.e. keep bins in shade.
  • If delivering to a packinghouse, minimize the time between harvest and delivery of fruit.

Cherry and apricot trees

Management of trees affected by bacterial canker/blossom blast during spring 2020

Bacterial canker and blossom blast, caused by Pseudomonas syringae pv. sryingae, were widespread this spring due to the April and May frost/freeze events. If you have affected stone fruit trees, particularly sweet cherry and apricot trees, prune any cankers from trees now, while the weather is hot and dry.

The bacterial population numbers are low this time of year; consequently, infection of pruning cuts will be minimized. Bacteria will be active in cooler months, such as the fall and spring. Affected limbs should be pruned several inches below the canker, such that an “ugly stub” remains to limit the spread into the tree. The causal bacteria can be transmitted by pruning tools, so these should be disinfected between prunings if bacterial canker is present. Additional management strategies for the fall will be described at a later date.

(Left) ) Prune out bacterial cankers from cherry trees during hot dry conditions. (Right) Now is the time to prune out cankers that occurred due to early-season infection by Pseudomonas syringae pv. syringae. Photos: Kari Peter, Penn State

Kari A. Peter, Penn State University

Photo at top: Protect apples from late season fruit rots. Photo: Kari Peter/Penn State

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