BYGLers are reporting that four unrelated springtime problems are causing some oaks in Ohio to look pretty ugly. These include the oak shothole leafminer (Japanagromyza viridula) with the flies producing holes, necrotic tissue, and missing leaf parts; oak anthracnose with the fungal disease producing necrotic tissue; and leaf blisters produced by either a fungal disease or an eriophyid mite.
None of these problems cause harm to the overall health of their oak hosts. However, singly or collectively, they certainly affect the tree's aesthetics.
Damage produced by the oak shothole leafmine was described in a BYGL Alert earlier this season titled, "Holy Oaks." You can read the full Alert by clicking on this hotlink:
Holes produced by the female flies in newly expanding leaves or nascent leaves furled in the bud remain evident through the season with the holes becoming larger with leaf expansion. Leafmines produced by the larvae (maggots) are at first light green but turn dark brown once the fly maggots complete their development. The necrotic tissue commonly crumbles away leaving large voids in the leaf.
Although the name of the genus may imply this is a non-native fly, apparently it is a native fly that is grouped in a genus that also includes several Asian species. However, in recent years, this fly has been behaving in Ohio like a non-native with populations expanding apparently unchecked year-after-year.
We're not alone. Using an entomology listserv, I asked entomologists located around the county what they've been seeing with the shothole leafminer. Several reported that damage was common last year in New England and the Mid-Atlantic States as well as Indiana and Kentucky. Most reported they're seeing a repeat this season.
Oak anthracnose is produced by the fungus Apiognomonia quercina. The oak anthracnose fungus is host-specific; it only infects oaks. The fungus does not infect sycamores to produce sycamore anthracnose, and vice versa. This is also true for other fungal pathogens that produce anthracnose diseases on maple, ash, and beech. Each is specific to a single host type.
A common symptom of oak anthracnose is brown, necrotic tissue that appears to expand from leaf veins. The necrotic tissue may be bounded by a zone of chlorosis which is possibly associated with a defense reaction to fungal infection.
Localized infections occur on newly expanding leaves with the infections restricting normal leaf expansion. This can cause the leaf area around the infected tissue to become curled or distorted.
Cool, wet conditions support fungal infection meaning that oak anthracnose is more common during seasons with cool, wet springs. Typically, symptoms are more prevalent on leaves growing on lower branches owing to less air circulation.
This appears to be a particularly "good year" for oak anthracnose with symptoms remaining very evident. Fortunately, the environmental conditions conducive to infection typically fade away by the time we see further leaf development in late-spring. This means the symptoms will likewise fade away as the season progresses.
Blistered Oak Leaves: Fungus or Eriophyid Mite?
Symptoms of the disease known as oak leaf blister caused the fungus, Taphrina caerulescens, are appearing on oaks in southwest Ohio. Disease symptoms include irregularly-shaped bulging blister-like spots that occur randomly on leaves or are clustered together to produce leaf distortion.
The blisters may be found on the upper or lower leaf surfaces with matching pocket-like depressions on the opposite sides of the leaves. The affected tissue is light-green to yellowish-green in early summer, but will eventually turn light brown to brownish-black.
Like oak anthracnose, fungal infections occur in the early spring on newly expanding leaves and is favored by cool, wet environmental conditions. Infections have been recorded on over 50 different species of oak belonging to both the white oak and red oak groups.
Look closely at leaves with blister-like symptoms on the upper leaf surface. Hair-like growth appearing in the corresponding depressions on the lower leaf surface are the handiwork of the eriophyid mite, Aceria triplacis (family Eriophyidae).
The eriophyid mite does not have a common name approved by the Entomological Society of America. However, given the number of times this mite has fooled me into believing I had found oak leaf blister, I've chosen to call it the oak leaf blister mite.
Oak leaf blister mite may be found on several members of the white oak group, but I've most often found it on burr oak (Quercus macrocarpa) and white oak (Q. alba). They live and feed within the hair-filled pockets. Of course, don't expect to see spider-like mites if you look closely with a 10x hand lens. As with all eriophyid mites, you have to look very closely using at least 40x magnification to see these odd-looking cigar-shaped arachnids.