Most crabapples look gorgeous in the early spring and go down-hill from there. As early as May one can see drap olive-colored spots developing on the leaves. By mid-August half of leaves may have dropped while the other half are shrunken, distorted, spotted, and downright ugly. The culprit is a ubiquitous fungus called apple scab. GreenStreet can easily prevent it with two fungicide sprays applied when the leaves first emerge.
A couple points to consider: not all cultivars of crabapple are susceptible to apple scab. Many of the modern cultivars have been selected for disease resistance — it’s the old-fashioned ones that are most seriously infected. Also, perhaps surprisingly, the disease doesn’t do much damage to a tree’s overall health. Most of any tree’s growth is done very early in the season, and if all of a crabapples’ leaves are lost in mid-summer, it can flower and leaf-out the following year.
With respect to control, timing of treatments is important. Fungicides work mostly as prophylactics and can’t kill a pathogen once the pathogen has invaded plant tissues. Also, it’s not possible to eliminate the problem. Spores are everywhere in early spring, and they travel considerable distances in the winds. The essential aim is to protect the vulnerable succulent developing plant tissues from infection.