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Getting the Upper Hand on Tree Fruit Diseases in July

Posted: July 18, 2017

Conditions may be optimal for several fungal and bacterial pome and stone fruit diseases in July.
Postharvest fungicide sprays are necessary to prevent defoliation from cherry leaf spot. Photo: K. Peter, Penn State

Postharvest fungicide sprays are necessary to prevent defoliation from cherry leaf spot. Photo: K. Peter, Penn State

Disease management becomes critical as harvest nears, and a review of the strategies for keeping fruit free from pathogens is always a good idea. As a reminder: 2 inches of rain washes off fungicides, so keep an eye out for those middle of the summer storms.

Brown Rot Management Strategies: Peaches and Nectarines

Peaches and nectarines are ripening, which means July is the season for brown rot. The fungus causing brown rot is quite opportunistic: it can kill blossoms and it can also ruin the fruit you’ve worked hard all season to grow. Brown rot disease is favored by warm, wet weather conditions. 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. It’s not uncommon to have brown rot appear “overnight” on fruit.

Spores produced on early maturing cultivars can fuel a continuing outbreak on late maturing cultivars—this is especially important for those who have already battled rot infections during the 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’s critical to be on top of insect management. Another concern to worry about is split pit. Unfortunately, these fruit are quite prone to rot problems. Keep in mind: 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 exceeded 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:

  • Luna Sensation (FRAC Group 7 + 11; 1 day PHI)
  • Luna Experience (FRAC Group 7 + 3; 0 day PHI)
  • Topsin M (FRAC Group 1)
  • Inspire Super (FRAC Groups 3 + 9; 2 day PHI)
  • Orius (FRAC Group 3; 0 day PHI)
  • Tilt (FRAC Group 3; 0 day PHI)
  • Quash (FRAC Group Code 3; 14 day PHI)
  • Gem (FRAC Group 11; 0 day PHI)
  • Captan (FRAC Group M4; 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. Depending on the number of sprays needed and what you may have used during bloom time, be sure to practice fungicide resistance management and rotate chemistries by FRAC group (“Spray by the Numbers”).

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 often, and prompt removal of infected fruit as soon as you see it. Spraying often ensures you have continuous protection; removing infected fruit from the trees ensures you are decreasing the amount of spores available to cause disease and hopefully minimizing an epidemic. Knocking infected fruit to the ground will be enough to limit spread. Vigilance is important and 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 Opti, and Regalia.

Cherry Leaf Spot: Postharvest Fungicides Needed

Cherry leaf spot (CLS) is similar to apple scab when it comes to infection conditions: warm and wet. The cherry leaf spot fungus prefers moderately wet conditions (hours of leaf wetness), with temperatures above 60°F. Although growers may be at the end of their tart cherry harvest, disease management is still necessary postharvest.

When conditions are favorable for cherry leaf spot, growers will observe one too many yellowing leaves in tart cherry orchards and are encouraged to apply two postharvest fungicide applications to prevent cherry leaf spot infection. The goal for the remainder of the season is to keep those leaves on the trees and prevent premature defoliation due to CLS infection. Tart cherry trees should not be bald in August as this will stress trees, setting them up for a weakened condition as they enter the winter months. Many options are available, such as Bravo, Indar, Merivon, and Syllit (to name a few).

Favorable Conditions for Sooty Blotch, Flyspeck, and Rots

Depending on your location, the management threshold for sooty blotch and flyspeck (SB FS) occurred approximately during the latter half of June. As the season continues, summer cover sprays including Captan, Topsin M, Ziram, and strobilurins (Flint, Sovran, Merivon, Luna Sensation) will provide excellent protection. However, I strongly encourage growers to reserve applications of Merivon and/or Luna Sensation until closer to harvest in order to mitigate fruit rots that can pop up in storage.

Fruit rots typically manifest in July and August and fruit susceptibility increases as it begins to mature. The optimum conditions for disease development include rainfall, relative humidity of 80 to 100%, and warm temperatures (80–90 °F). Fruit rot disease management is most effective by applying fungicides on a 10– to 14–day interval schedule through harvest; more frequent under favorable conditions. Captan and Ziram provide good protective control and are most optimal when combined with another fungicide. Strobilurins (Flint, Sovran) are excellent for controlling fruit rot. To reduce the risk of resistance, alternating a fungicide with another FRAC code and tank mixing are encouraged.

There are a few alternative options available for managing summer diseases, in addition to sulfur. In 2015 and 2016, we saw excellent SB FS control using Oso (polyoxin D salt; Certis). For several seasons in a row, we used Serenade Optimum (Bayer) during the last two cover sprays and it showed promise managing fruit rots later in the season and even in cold storage (disclosure: we used a conventional program up until sixth cover).

In addition, Cueva and Double Nickel (Certis) has also shown some control for summer disease management. It is very important for those using alternative options to be vigilant about keeping up with management. This means closely monitoring the weather, as well as spraying more often when infection conditions persist. For the 2017 season, we are evaluating other alternatives and may have more options for growers in the near future.

Bacterial Spot: Favorable Weather Conditions Persist

Conditions have been favorable lately for bacterial spot on peaches and nectarines and symptoms are afoot in the orchard on susceptible cultivars. Shorter application intervals should be used when rainy periods are frequent and temperatures range from 75°F to 85°F. A longer 14-day interval is acceptable during extended periods of dry weather. Management options include oxytetracycline (MycoShield, FireLine; 21 d PHI) and copper. Coppers vary in their PHI, so use products accordingly when you are nearing harvest.

When using copper, remember slow drying conditions and pH of the solution will influence copper injury. In addition: be mindful of what you tank mix with your copper—NO foliar fertilizers and beware of any product that will make the solution more acidic.

Other controls to consider for incorporating into a management rotation are Serenade Opti (14 oz/A; Bayer) and Double Nickel (2 qt/A; Certis). Last year, we evaluated Regalia (2 qt/A; Marrone Bio Innovations) and observed control on varieties that had low to moderate susceptibility; however, on very susceptible varieties (example: Snow King, Sweet Dream), Regalia showed little bacterial spot control.

Apple Scab: Primary is Finally Over

A disease update is not complete unless there is at least one mention of apple scab. It may be kind of odd to be talking about apple scab in the middle of July, but 2017 has turned out to be an odd year.

Primary scab infection is defined as the ascospores released from the overwintered leaves from the previous year’s infected leaves. The week of July 10, 2017 was the first week where we did not detect any ascospores from our overwintering leaves. We figured this has to be one of the longest primary scab periods ever: We started detecting the first spores March 6, 2017 and the last spores the first week of July, 2017.

If you made it through the primary infection period with no scab to be seen: Congratulations! However, it’s important to continue to scout your orchards since we had a few good infection periods during the down swing of the 2017 primary period. Whatever spores established on leaves and/or fruit during the primary infection period will produce additional spores called conidia in vast quantities.

These summer spores can cause infection throughout the summer and we call this stage the “secondary infection period.” It is important to monitor your orchard for any scab infection that became established during the “primary” period because scab control will be needed throughout summer in order to keep the disease from causing significant damage to fruit, especially if we continue to experience weather conditions ideal for infection.

Reminders

Using sulfur during hot weather to manage tree fruit diseases

Sulfur is phytotoxic at high temperatures and should not be sprayed when temperatures at time of application or predicted for the next several days will exceed 85°F.

Bacterial canker on cherry: Prune trees in summer during dry weather

For any folks who struggle with bacterial canker infections in their cherry trees, summer is the best time to prune your trees, particularly during dry weather. The bacteria do not like hot, dry conditions and the pathogen population will be at its lowest. Research out of Cornell showed no benefit of copper applications before and after pruning. Save copper sprays for the fall and early spring when cool, wet weather favor bacterial populations to grow and trees will be the most vulnerable.

Summer storms mean potential for hail

The afternoon of July 17, 2017, a whopper of a storm blew through Adams County producing a large amount of hail. Fire blight can still rear its ugly head post terminal bud set when trauma events, like hail, occur. Remember to apply streptomycin within 24 hrs of a hail event to prevent trauma blight.

If you experienced a catastrophic loss due to hail, diseases still need to be managed to keep pathogens in check for next year. Open wounds on growing shoots, branches, or scaffolds are entry points for bacterial and fungal pathogens (as well as insects). As far as the rest of the season, growers should maintain a minimal pest management program to protect trees during the wound healing process and not predispose trees to further damage, otherwise long-term health, productivity and longevity of the orchard may be severely compromised.

Contact Information

Kari A. Peter
  • Assistant Professor
Email:
Phone: 717-677-6116