2022 Disease Update: Management for Sooty Blotch and Flyspeck, Fire Blight, and Bacterial Spot

Management strategies for controlling sooty blotch and flyspeck, bacterial spot on stone fruit, and monitoring for fire blight infections are discussed below.

Sooty Blotch and Flyspeck Management

Sooty blotch and flyspeck (SBFS) are summer diseases that can cause headaches for apple growers. The diseases often occur concurrently on the fruit resulting in surface blemishes that detract from fruit appearance, lowering fruit quality and market value. Sooty blotch also shortens fruit storage life because of increased water loss.

The fungi causing the diseases overwinter on the twigs of many woody plants and apple and pear. Spores of the fungi are windblown into and throughout the orchard; fruit infection can occur any time after petal fall but is most prevalent in mid-to-late summer. Several disease models are variations on the original model published by Brown and Sutton (1995) to predict sooty blotch and flyspeck infection periods. For the Brown and Sutton model, leaf wetness hours greater than four hours starting 10 days after petal fall are counted. The threshold to start treatment begins approximately at 220 hours of leaf wetness. For the 2022 season, we met this threshold within the last two weeks.

Disease outbreaks are favored by extended periods of above-normal summer temperatures combined with frequent rainfall and high humidity. These diseases usually appear on fruit late in the season. New infections can be observed as late as September. Regular captan (2-3 lb/A) applications are enough to manage SBFS. However, if frequent rain events occur, growers are encouraged to add Topsin M (1 lb/A; 1-day PHI) to captan since captan can be washed off. The addition of Topsin will provide extra protection during very wet periods.

Organic options that have some efficacy include sulfur, Oso (4.5 fl oz/A), and bacterial-based products (Serenade, Double Nickel).

Fire Blight Management

Symptoms are now obvious for fire blight. Consequently, it is imperative growers begin scouting their orchards regularly, especially on newly planted trees. The earlier fire blight infections are caught, the easier it is to manage the disease.

Figure 2. Fire blight is best cut out early. Early symptoms (darkened regions) are typically observed in the mid-vein of the leaf. Photo: K. Peter, Penn State

Items to keep in mind when pruning out fire blight infections

• Do not cut out infections during wet weather since bacteria move via water.
• Cut out active infections early - before necrosis develops (limits the spread of bacteria).
• Pruning is most effective when incidence is low.
• Focus on salvaging tree structure and young high-density plantings when incidence is high.
• Bacteria can invade healthy tissue up to ~3 feet in advance of visible symptoms, which makes tool sterilization ineffective.
• Practice the ugly stub method: cut 6 -12 inches below the margin of visible infection and remove later during winter pruning.
• Ideally, infected tissue should be removed from the orchard and burned. However, if pressed for time, prunings can be left in the row middles to dry out and subsequently chopped with a mower.

If you observe fire blight, consider using the “Actigard Paint" method

• After cutting out the fire blight, apply a high concentration spray of Actigard to the cut and 1 meter of trunk below the limb with the fire blight. According to the label, mix 1 oz Actigard in 1 quart of a 1% penetrant. The penetrant suggested on the label is Pentrabark (an organosilicone); however, a similar penetrant can be used. Do not apply as a spray since the leaves will be adversely affected by the high concentration of the solution. One quart will treat approximately 500 cuts. Do not apply within 60 days of harvest.
• If an infection is evident, consider an application of the highest rate of ProCa (12 oz/A) to shut down the infection spread. Another option would be three applications of ProCa (6 oz/A) + Actigard 2 oz/A.

If it is a bad fire blight year, we recommend a “triage" method when it comes to pruning decisions once fire blight has struck, going from highest to lowest priority:

• Young orchards 3 – 8 years old with just a few a strikes. (highest priority)
• Young orchards 3 – 8 years old with severe strikes.
• Older orchards with a few strikes.
• The “walk away" group: orchards with so many strikes that most of the tree would need to be removed; severe pruning can stimulate new growth that can become infected. (lowest priority)

Folks have been told all along to prune out fire blight during the season when they see it; however, there can be too much of a good thing: It is very important to avoid excessive cutting when pruning out fire blight. Excessive cutting will encourage more shoot growth and worsen your fire blight problem. This is especially true for older orchards where cankers may be lurking about in the tree.

Bacterial Spot Management

Conditions are favoring bacterial spot on stone fruit. Management programs for susceptible stone fruit varieties (peach, nectarine, plum, apricot) should be underway and continue until about 3 weeks before harvest when the preharvest brown rot management program occurs. Use copper at 0.5 – 1 oz of metallic copper per acre and rotate with the following:

• Serenade ASO 4 qt/A
• Serenade Opti 14 oz/A
• Double Nickel 2 qt/A
• Regalia 2 qt/A
• Mycoshield/FireLine 12 oz/A

Important nuggets of wisdom to memorize when using copper on stone fruit trees

• Copper is a general biocide: Copper will kill fungi, bacteria, AND plant cells.
• Do not tank mix copper with a foliar fertilizer or phosphorous acid product (i.e., Rampart, Phostrol, Prophyt).
• Captan and sulfur will also cause phytotoxicity (symptoms will be like copper injury).
• Copper sprays will become more phytotoxic if they are applied in an acidic solution: beware of adjuvants that lower pH of tank mixes. Growers may add lime to reduce the potential for plant damage.
• Copper phytotoxicity worsens under slow drying conditions. Use alternative products during rainy periods.
• Copper injury causes round holes of different sizes and will appear like a spray pattern on the leaf. Defoliation can occur with repeated copper applications and is mostly observed on the oldest leaves. Bacterial spot symptoms appear as angular spots, following the veins of the leaf, resulting in yellowing and eventual defoliation.

Figure 3. Bacterial spot on stone fruit leaves manifest as angular lesions typically following the veins of the leaf. Photo: K. Peter, Penn State

If growers lost their crop due to the cold temperatures this spring, bacterial spot management is necessary since it can also cause premature defoliation. It is recommended that highly susceptible fruit trees without fruit be monitored for disease development on leaves and subsequent defoliation during optimal disease conditions, which are warm (75–85°F), wet, and windy. If the disease severity increases during the season, applications of oxytetracycline are recommended. If we experience a hot, dry summer, the progress of the disease will slow.

Come One, Come All!

Penn State FREC Grower Field Day, July 7, 2022

The Penn State Fruit Research and Extension Center will be hosting a Grower Field Day on Thursday, July 7, 2022. We will have morning demonstrations, a BBQ Chicken lunch, and afternoon sessions covering the latest research in ag engineering, horticulture, entomology, and plant pathology. For more registration information, please email or call Kristy Loper: kkl5535@psu.edu or 717-677-6116 Ext. 0.