Fruit trees are or will soon bud out. This is an important time to begin applying fungicides to trees that had a fungal leaf disease last season.
Apples and crabapples are often infected by apple scab and cedar apple rust. Both cause spots to develop on leaves followed by leaf yellowing and dropping throughout the season.
Neither disease will kill a tree in one or even a few seasons, but can reduce yield. For this reason, and because fungicides work best when applied just before and during the infection period, this spring is the time to treat trees that were diseased last season.
After a tree is infected, most fungicides do very little for control. When diseased leaf samples are brought to me in July and August, I tell homeowners to clean up and destroy fallen leaves but wait until the following spring to apply a fungicide.
If an apple or crabapple tree was infected last season, and the infection resulted in over 50 percent of the leaves dropping off in July and August, a fungicide applied this spring will reduce infections for this year.
There are resistant apple and crabapple cultivars. These trees rarely need to be treated with fungicides. If leaf spotting and heavy leaf drop has never occurred on an apple or crabapple, the tree is likely resistant and fungicide applications are not needed.
For susceptible trees, fungicide treatments during the spring following an infection year will help reduce disease. But timing is everything when it comes to using a pesticide.
The fungi that cause these diseases release spores and infect plants in April and May. This is when a fungicide will be effective. After leaf spots have developed, it is too late for fungicides to work well.
Just as tree buds begin to swell and green leaf tissue is showing is the time to begin applying a fungicide. Repeat application two to three times according to label direction, typically every 7 to 10 days.
The fungicide myclobutanil (Immunox) is available to homeowners and is effective in controlling apple scab and cedar apple rust. Note there are several formulations of Immunox but only one is labeled for fruit.
Before treating any plant, especially edible plants, check the label. The pesticide needs to be labeled for the plant being treated. If an edible, know what the recommended waiting period is between treating and when the fruit can be harvested and eaten.
If an apple tree also needs protection from insects like apple coddling moth, an insecticide can be added but wait until after blooming is finished and all petals have dropped off to avoid harming bees.
According to Ward Upham with Kansas State, Methoxychlor or malathion were insecticides used in the past for this but some of these products may no longer be labeled for fruit. Upham suggested a new homeowner product with the trade name Bonide Fruit Tree and Plant Guard.
This product also contains two fungicides. The active ingredient for insects is lambda-cyhalothrin. The fungicides are Pyraclostrobin and Boscalid. The fungicides only suppress cedar-apple rust and so are not as effective on cedar-apple rust as Immunox.
Immunox could be used for the first application as leaves begin to bud. Then Bonide used after petal drop. As always, protect bees and other pollinators by never applying any insecticide during blooming.
Source: Kelly Feehan, Extension Educator
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