2022 Disease Update: Possible Apple Scab Infection Event May 1-2

Growers are encouraged to use FRAC Group 7 fungicides during this period. Forecasted conditions may become favorable for fire blight May 3–5. As a reminder, the dry weather experienced in-between wetting periods favors powdery mildew in tree fruit. Management is needed for susceptible cultivars. Growers need to be vigilant during the coming week.

The number of overwintering apple scab spores at maturity are peaking. Combined with favorable conditions (moderate temperatures, a wetting event with at least six leaf wetness hours), we are in the period for the highest disease pressure for apple scab. The forecasted weather conditions favor a scab infection event on May 1-2. In addition, growers are to remain on high alert May 3 – 5 for potential fire blight conditions. We plugged in the forecasted temperatures (high and low) and rainfall into MaryBlyt to analyze conditions for the coming week. Although no infection events are predicted at this time, the current models predict an EIP (epiphytic infection potential) of 70 or higher. When the EIP is over 70, growers need to pay attention.

If the temperatures that manifest during this week are cooler than the prediction, the EIP will decrease; if they are higher, it will increase and an infection event may be triggered. In addition, the forecast for rain ranges from 30-to 70% during the week. We are experiencing stretches of dry weather between our rainy periods. As a reminder, these conditions favor powdery mildew in tree fruit.

Although no fire blight infection events are forecasted, the EIP is forecasted to be above 70 on May 4.

Apple scab management

Tank mix with a rainfast mancozeb. In addition to fungicide resistance management for apple scab, this will manage rust infections. Growers are highly encouraged to use complete sprays instead of ARM from bloom through petal fall, especially if frequent rain events favor extended wetness periods.

During this time, it is best to use FRAC Group 7, 7 + 9, 7 + 11 fungicides:

• Aprovia (FRAC Group 7; 5.5 fl oz/A)
• Excalia (FRAC Group 7; 3 – 4 fl oz/A)
• Fontelis (FRAC Group 7; 16 fl oz/A)
• Miravis (FRAC Group 7; 3.4 fl oz/A)
• Sercadis (FRAC Group 7; 4.5 fl oz/A)
• Luna Tranquility (FRAC Groups 7 + 9; 11.2 fl oz/A)
• Luna Sensation (FRAC Groups 7 + 11; 5 fl oz/A)
• Merivon (FRAC Groups 7 + 11; 5 fl oz/A)
• Pristine (FRAC Groups 7 + 11; 14.5 – 18.5 oz/A)

When necessary, rotate FRAC Group 7 fungicides with the following:

• Cevya (FRAC Group 3; 5 fl oz/A)
• Indar (FRAC Group 3; 8 fl oz/A)
• Inspire Super (FRAC Groups 3 + 9; 12 fl oz/A)
• Scala (FRAC Group 9; 5 – 10 fl oz/A)
• Vangard (FRAC Group 9; 5 oz/A)

Limit FRAC Group 7 and 11 fungicides to two complete applications during this period of high disease pressure. A maximum of four complete applications are allowed per year for FRAC Groups 7 and 11 fungicides for fungicide resistance management—regardless if you use a single mix or a pre-mix. Save two FRAC Group 7 and 11 fungicide sprays for summer and late-season to manage fruit rots.

Figure 2.

Fire blight management

As a reminder, the minimum requirements for blossom infection and the order in which they must occur are:

1. Flowers must be open with petals intact (flowers in petal fall are resistant)
2. An accumulation of at least 198-degree hours above 65ºF
3. A wetting even as dew or rain
4. An average daily temperature of 60ºF

Consider applying products that activate plant defenses to your fungicide sprays to help get a jump start on management, especially since it takes approximately 48 hours to observe the product's benefit. We have observed an additive effect of repeated applications of plant defense activators (applying every 3-5 days).

Streptomycin remains the best option for management. Do not use ARM sprays; complete sprays will avoid an unintentional infection trigger, especially under the current conditions.

• A typical streptomycin application gives you 48 hours of protection.
• When applying streptomycin, you are killing the bacteria within the flower in that moment and resetting the clock.
• For added protection, include a plant defense elicitor (Actigard 1- 2 oz/A) or Apogee/Kudos (2 – 6 oz/A) in your streptomycin application. LifeGard and Vacciplant are additional options.
• Organic management options include Blossom Protect or Regalia 1 qt/A plus Magna-Bon CS2005 1 pt/A. Blossoms must be open and these products need to have been applied prior to the anticipated infection event to be effective.

Powdery mildew management

The powdery mildew fungus favors temperatures from 50°F to 77°F and relative humidity above 70%.

Apple powdery mildew

• Infection can occur on both leaves and fruit.
• If conditions are dry, focus on managing apple powdery mildew by using products such as Rally, Topguard/Rhyme, Trionic/Procure, Torino, sulfur, or potassium bicarbonate (Kaligreen, Mil Stop, Carb-O-Nator). Torino can be used once per season on apple. In 2021, we experienced many dry days during the early season, and powdery mildew caught some people off guard. Although dry weather favors low apple scab disease pressure, this is not the case for powdery mildew. Please do not forget about powdery mildew during those dry days. Some strong powdery mildew products are not as effective against scab; however, tank mixing with a broad-spectrum fungicide (i.e., mancozeb) will keep the scab in check when we have rain events in the middle of dry stretches. For those using low-risk programs, sulfur and potassium bicarbonate can control powdery mildew and scab.

Rusty spot on peach/nectarine

• Infection occurs only on fruit, and symptoms will first appear 2-3 weeks after shuck split.
• The time to apply fungicides for rusty spot on growing peaches and nectarines is petal fall, shuck split, first cover, and the second cover.
• Fungicides to use during this period are Rally, Rhyme, Inspire Super, Flint Extra, Merivon, Luna Sensation, Luna Experience, Orius, Fontelis, and sulfur.

Figure 3. Prevent rusty spot on peach with fungicides during petal fall, shuck split, first cover, and the second cover. Photo: K. Peter, Penn State

Commercial fruit growers, please note

When controlling for disease, weather and tree growth conditions need to be monitored at a local level within one's own orchard. To assist with management decisions (especially infection events), growers can use the NEWA website, which has weather stations all over Pennsylvania. Before chemical products are applied, be sure to comply by obtaining the current usage regulations and examining the product label. Product information can be easily obtained from CDMS.