Jul 16, 2015
Disease Update: Rots wreaking havoc on stone fruit

The extremely wet weather over the last month has triggered rot issues in both sweet and tart cherries. As a result of the volume of fungal spores flying around and the persistent warm wet conditions, peach and nectarine growers need to be on high alert as we are nearing the home stretch for harvest. Vigilant rot management strategies are critical this year to prevent brown rot (and other rots) during preharvest and postharvest. Management options, including organic strategies, are discussed. Conditions continue to be favorable for bacterial spot, necessitating shorter spray intervals for disease management.

The rots are running amok thanks to rainforest-like conditions we have been experiencing the last many weeks. Although this is a plant pathologist’s dream to have such wonderful conditions for disease, I am not jumping up and down for joy: with the exception of ice cream, you can have too much of a good thing. We are at the point at FREC where we are counting the number of dry days. We hit a streak recently: 4 days, July 10 – 13. That streak was ruined yesterday morning. Needless to say, it has been difficult to keep up, and I know the vast majority of the readership feels the same way. The amount of rain we have received has caused stone fruit to balloon making fruit skin weakened and cracked, thereby providing easier access for fungal spores. Cherries took a major beating this year with brown rot, as well as Alternaria rot (see picture). With such high spore pressure and optimal disease conditions, peaches and nectarines are under a significant threat of rot diseases as we are on the doorstep of harvest. In Penn State’s research orchards, I have been finding brown rot already on Red Haven, Sunglo, and Easternglo…so far

Brown rot: This is going to be a bad year

The fungus causing brown rot is quite opportunistic: it can kill blossoms and it can also ruin the fruit you have worked hard all season to grow. Brown rot disease is favored by warm, wet weather, which has been occurring daily since mid-June. Under optimum temperature conditions, fruit infections can occur with only three hours of wetness when inoculum levels are high. Longer wet periods during infection result in shorter incubation times so symptoms develop more rapidly. I’ve heard reports that folks saw brown rot appear “overnight” on their fruit. This is not science fiction: this can happen.

Spores produced on early maturing cultivars can fuel a continuing outbreak on late maturing cultivars – this is especially important for those who have battled rot infections already this season. To add another headache to the issue, insects can be important vectors of the fungal spores during fruit ripening: they can carry spores to injury sites produced by oriental fruit moth, Japanese beetle, green June beetle, and other insects that can injure fruit. Wounded fruit are much more susceptible to brown rot than unwounded fruit. It is critical to be on top of insect management.

Keep in mind that under the right conditions, “healthy” fruit harvested can be contaminated and may decay later during storage. Research at Rutgers has shown that timing brown rot sprays 18 days, 9 days, and 1 day before harvest provided greater than 95 percent control under heavy disease pressure. When following this regime, be sure to rotate chemistries by FRAC Group Code number for resistance management. For example, one could spray the following (provided the maximum number of sprays has not been met for that chemistry):
18 days: Fontelis (FRAC Group 7; 0 day PHI)
9 days: Indar (FRAC Group 3; 0 day PHI)
1 day: Merivon (FRAC Group 7 +11; 0 day PHI).

Other options to rotate:
Topsin M (FRAC Group 1) + Captan (1 day PHI)
Inspire Super (FRAC Groups 3 + 9; 2 day PHI)
Orius/Elite (FRAC Group 3; 0 day PHI)
Tilt/Orbit (FRAC Group 3; 0 day PHI)
Quash (FRAC Group Code 3; 14 day PHI)
Gem (FRAC Group 11; 0 day PHI)

Keeping in mind products that were used to control blossom blight, be sure to be in compliance by obtaining the current usage regulations and reading the product label.

Other rots for you radar: Alternaria rot, Rhizopus Rot, Anthracnose

Alternaria rot reared its ugly head this year on cherries – for some more than others. Alternaria rot manifests as velvety dark green to black, circular, sunken lesions on mature fruit. Alternaria needs a weakened area as an entry point, typically over ripe fruit are most susceptible; however, the volume of rain around we received around the cherry harvest probably exacerbated (skin splitting/cracking) the disease incidence this year. The fungus has also been reported to cause superficial red spotting on the surface of apricots and peaches. The spots eventually turn tan to brown, becoming necrotic, but typically retaining a red halo. Unfortunately, there are no practical control recommendations as evidenced by labels not including “Alternaria rot.” However some chemicals, such as Merivon, include controlling “Alternaria leaf spot (Alternaria spp.).” If one is using such a chemical to control for brown rot in their orchard, there is a good chance a rot caused by an “Alternaria spp.” will also be controlled.

Rhizopus rot and Anthracnose are also favored by warm, wet conditions. Rhizopus rot causes the skin to slip readily from the decaying flesh underneath, while other rots do not. Early on, the fungus will appear as a white fluffy mass, which later turns dark gray to black as the fungus begins to sporulate. Anthracnose on peaches and bitter rot on apples are caused by the same fungus. As a result, the symptoms are similar: small, brown spots which become darker, circular and slightly sunken as they age; larger, sunken anthracnose lesions are firm to the touch and are often covered with concentric rings of salmon-colored spore masses. Both of these rots favor ripe or nearly ripe fruit. Like Alternaria, these rots can occur during preharvest, as well as postharvest. Chemicals used to control brown rot will also manage these rots.

Bacterial Spot: Optimal weather conditions favor shorter spray intervals

Bacterial spot in my research orchards has exploded the last few weeks and I know folks are struggling. Complete sprays and shorter application intervals should be used when rainy periods are frequent and temperatures range from 75°F to 85°F. Keep in mind the preharvest interval for products you choose to use. If choosing to use copper, be aware the slow drying conditions we have been experiencing will worsen phytotoxicty symptoms on leaves. For more information about options, refer to the May Fruit Times article. Other controls to consider for incorporating into a management rotation are Serenade Optimum (Bayer) and Double Nickel (Certis).

Alternative options for rot management

The key for growers who farm organically or prefer using alternative products is to spray as often as possible as disease conditions persist, manage insects, scout regularly, and promptly remove infected fruit as soon as you see it. Since brown rot can pop up overnight, scouting daily is most likely necessary. Spraying often ensures you have continuous protection; removing infected fruit (don’t throw fruit on the ground – remove from the orchard block) ensures you are decreasing the amount of spores available to cause disease and hopefully minimizing an epidemic. This may translate spraying every few days, especially if rain washes off products. According to studies at Rutgers, sulfur is not effective for controlling brown rot. Some organic options labeled for brown rot control are Cueva, Double Nickel, Serenade Optimum, and Regalia (Marrone Bio Innovations). Anecdotally, I have heard combining more than one of these products in an application provides better control than a single product alone. (Please note: these products can also be used in conventional management programs, as well, in tank mixes or rotations.)

— By Kari Peter, Ph.D., Assistant Professor, Penn State University

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