First Symptoms of Sooty Blotch & Flyspeck and Apple Blotch Disease Visible in Virginia: 22 July 2021

On 22 July we detected the first signs of sooty blotch and flyspeck (SB&FS) on an unknown apple cultivar fruit in Appomattox VA (Fig. 1). This is visible, of course, on fruit that were not treated with any fungicides during spring or summer and likely the advanced development stage indicates that fruit exhibited these symptoms for 5 to 10 days before we detected them. Depending on your spray schedule and used fungicides, we hope you do not see this disease on your fruit. However, start scouting your orchard to check for absence or presence of SB&FS symptoms now to see how well was your fungicide coverage during the cover sprays.

FIGURE 1. FIRST SIGNS OF FLYSPECK AND SOOTY BLOTCH ON FRUIT OF UNKNOWN APPLE CULTIVAR IN APPOMATTOX VA (PHOTO BY ACIMOVIC 2021). CLICK ON THE PHOTO TO ZOOM IN.

In 2021, the weather conditions during spring and through the beginning of summer 2021 were highly favorable for Apple Blotch Disease (ABD), also known as Marssonina Leaf Blotch (MLB), to infect and express on apple leaves. On 22 July, we detected the first symptoms of this apple disease caused by the plant pathogenic fungus Diplocarpon coronariae on the leaves in the lower part of the crown of an unknown apple cultivar (Fig. 2). In this orchard, which was not sprayed with fungicides at all during the spring and until now, the leaves are starting to yellow and drop. This disease started expressing in apple orchards in the East coast from the late summer 2017. It is primarily developing during the summer and is a different disease from apple scab, which primarily develops in spring and early summer. Many cultivars are susceptible to this disease: 'Honeycrisp', 'Stayman', 'Rome', 'Jonagold, to name just few of them. We assume that ABD will express only in orchards that had too long spray intervals between cover sprays in the last month and a half, while rain was occurring during these periods. This is the usual time to see MLB in orchards in Virginia and this disease will continue to express where fungicide coverage was not perfect. We must assume that this disease is here to stay and create problems during the summer in years to come, which implies that you have to apply your cover sprays depending on the amount and frequency of rain you get during the summer and keep cover sprays tight in locations where in the last two years you had issues with this disease (leaf blotches, yellowing and drop). Diplocarpon coronariae can develop resistance to fungicides that are effective against it, if one fungicide group is overused and fungicides from different groups are not alternated. 

FIGURE 2 A, B. ADVANCED APPLE BLOTCH DISEASE SYMPTOMS VISIBLE IN APPOMATTOX VA; BROWN PATCHES AND ENSUING YELLOWING JUST BEFORE LEAF DROP ON LEAF OF AN UNKNOWN APPLE CULTIVAR (PHOTO BY AĆMOVIĆ S. G., 2021). CLICK ON THE PHOTO TO ZOOM IN.

The first infections by this fungus usually start sometime in May or around the first or second week of June. We looked at the very useful  infection prediction model for Marssonina coronaria, which is a different name for the same pathogen, available through RIMpro and found that the first seasonal infection of this disease occurred on 31 March 2021 (Fig. 3) with the more infections in April and May. Infections on leaves are still occurring now with every rain event we get, unless the leaves were protected with a fungicide. 

FIGURE 3. OUTPUT OF THE Diplocarpon coronariae PREDICTION MODEL IN RIMPRO FOR WINCHESRE VA 2021 (SCREENSHOT BY PERMISSION OF RIMPRO B.V. NETHERLANDS: RIMpro, 2021).

We recommend growers to subscribe to RIMpro via https://rimpro.eu/ to use a very good apple scab prediction model and for the same cost use the Marssonina coronaria prediction model and plethora of other apple and grape disease and pest models in this service. Typical ABD symptoms express 40 – 45 days after the infection. There currently are no labelled fungicides for control of ABD on apple in Virginia but it is obvious that various fungicides are effective as in many orchards we do not see this problem or it is minimal. ABD can severely affect both conventional and organic apple orchards, especially when reduced spray programs or ineffective fungicides are attempted in a year with very frequent rains during the summer. If left unmanaged, ABD leads to defoliation of apple trees during late summer (again, the key is that cover sprays with fungicides are applied at shorter spray intervals when very rainy periods occur, i.e. they are applied in alignment with frequent rains events). ABD defoliation can weaken the apple trees and cause late fall tree re-growth, further delaying tree preparation for dormancy. Finally, ABD symptoms can occur on fruit after leaves were severely infected. The fruit are not marketable and are more frequently seen on late maturing cultivars ('Rome', 'Jonagold'). Typical symptoms of fruit are visible in (Fig. 4).

FIGURE 4. TWO APPLE BLOTCH DISEASE SPOTS ON FRUIT OF APPLE CULTIVAR 'ROME' CAUSED BY FUNGUS Diplocarpon coronariae (PHOTO BY ACIMOVIC S. 2019). ON THE SAME FRUIT FLYSPECK IS VISIBLE AS SMALL BLACK BUT CLUSTERED AND NUMEROUS DOTS.