Dieback issues with McIntosh cultivars

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

There have been several reports of dieback in new plantings of ‘McIntosh’ cultivars in the past couple of seasons. The cultivars observed by District Extension Educators throughout Michigan with this issue have primarily been ‘Linda Mac’ and ‘Pioneer Mac,’ and there may be others. In other apple growing areas in eastern North America, similar dieback is being reported in other Mac cultivars as well as ‘Northern Spy.’ All are on dwarf rootstock plantings.

While no one has been able to isolate a single causal agent, several theories have been bantered about. There does seem to be a particular age of tree that is affected – 4th or 5th leaf and having just had their first decent cropload. Also, it seems to be worse following a season with extended growth into the fall that begs us to consider the possibility of winter injury that sets these trees up for infections from opportunistic fungi such as Black Rot (Botryosphaeria obtusa), White Rot or Bot Rot (Botryosphaeria dothidea), Leucostoma (Cytospora), Alternaria, and Anthracnose. These fungi typically move into damaged tissue to cause further rot, but alone they do not usually cause stem cankers. Also, infections seem to follow a rainy, wet fall season. Many experts have tried to culture and inoculate to replicate this type of infection with little success so far, and the results just don’t seem to point to one organism. At this time, we need to focus on trying to properly identify the causal organism so some testing will be going on this summer through George Sundin’s lab at MSU. Contact your District Fruit Educator if you are seeing this kind of injury or for more details on sampling.

Linda Mac dieback
Photo 1. Linda Mac dieback.

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