Apple Disease Control: Bloom Through Petal Fall

By Kari A. Peter

The 2020 season is shaping up to be very interesting, as well as challenging. Based on our current conditions and the forecast, we may experience a protracted bloom this season for a large part of the apple growing region.

With that said, growers need to be on alert since this is the time frame for many important diseases to do damage. The following are diseases for the grower radar during bloom through petal fall and their management strategies.

Apple Scab and Marssonina Blotch

Apple scab spores are peaking (the maximum number of available spores dispersing from the overwintering leaves) during bloom through petal fall. April 23 – 26 will be an important apple scab infection event. Growers should remain on high alert the week of April 27 if the forecasted rain events manifest.

Marssonina blotch has been wreaking havoc the last several years, defoliating trees prematurely. The disease is like apple scab, such that spores overwinter in the previous year’s leaves. The timing for protecting trees from Marssonina has not been determined for Pennsylvania. To date, the information from the literature does not quite line up with what we are observing in Pennsylvania. Consequently, we are monitoring for Marssonina spore dispersal this season and hope to have a better answer in a few months. However, our preliminary data from 2019 suggests bloom through petal fall is an important time (like apple scab) and FRAC Group 7 fungicides work the best.

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)
• 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)

As a reminder: Tank mix fungicides with a rainfast mancozeb. If rain persists during this important period, please keep coverage on your trees, even if that means spraying in the rain. Avoid spraying during heavy rain events; light rain or misting is ideal. Use broad-spectrum fungicides when spraying in the rain: mancozeb, ziram, or sulfur.

Bitter Rot

We have preliminary data suggesting bloom could be a critical time to start bitter rot management. During our bitter rot studies the last few years, we have discovered there are multiple species of Colletotrichum causing bitter rot in Pennsylvania. After evaluating them for fungicide sensitivity, these fungicides are your best bet for management during bloom through petal fall:

• Aprovia (FRAC Group 7; 5.5 fl oz/A)
• Fontelis (FRAC Group 7; 20 fl oz/A)
• Merivon (FRAC Groups 7 + 11; 5 fl oz/A)
• Pristine (FRAC Groups 7 + 11; 14.5 – 18.5 oz/A)

Fire Blight Management

Many PA growers have been experiencing bloom since mid-April. Fortunately, the cool temperatures are not favoring fire blight development.

As a reminder for 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

If a protracted bloom occurs, it is important growers remain vigilant. There are many NEWA stations in Pennsylvania and growers can refer to the nearest one to their location to double-check the risk for fire blight.

For growers in Pennsylvania whose bud development is lagging behind the southern half of the state, please review strategies for early season disease management outlined in the March 26, 2020, Fruit Times 2020 Disease Update:

2020 Disease Update: Optimizing Apple Scab and Fire Blight Management