Plant and Insect Diagnostic Clinic Update – June 23, 2017

Symptoms and signs of willow leaf spot cause by the the fungi Monostichella salicis
Symptoms and signs of willow leaf spot caused by the fungi Monostichella salicis. Top: leaf spot (symptoms), center fungal bodies (signs), bottom spores (signs within the fungal bodies). photos by Lina Rodriguez Salamanca

The following are highlights and updates about samples and questions recently received in the Plant and Insect Diagnostic Clinic. Visit the PIDC's Facebook page to ask questions and for updates and more pictures. For more information on a particular disease or insect problem listed, follow the article cited.


The following plant disease highlights represent recent sample submissions from fruit, vegetables, and ornamentals.  

Broadleaf Trees

Willow, Maple and Oak anthracnose, see our encyclopedia article Anthracnose on shade trees

Oak powdery mildew, see our encyclopedia article Powdery Mildew

Maple suspected decline, see our encyclopedia article Tree decline

Willow leaf spot

Oak tatters, see our encyclopedia article Oak Tatters

Coniferous Trees

Spruce- Stigmina needle cast

Spruce- Sudden Needle Drop or Spruce Needle Drop (SNEED), IL clinic article on SNEED

Perennials and Annuals

Hosta virus X , see our encyclopedia article Hosta-infecting Virii

Fruit (small and tree fruit, including hops)

Apple - Fire blight, see our encyclopedia article Fire Blight



Broadleaf trees

We have gotten a several samples of trees with lecanium scale insects (two hackberry and a maple).  This type of scale has a wide host range and can feed on many different tree species, but will tend to be worse on stressed trees. Lecanium scales produce a sticky honeydew that will lead to sooty mold causing branches of infested trees to be a bit black colored. Lecanium scale crawlers can be treated with an insecticide labeled for scale control when they are active, mid-June through mid-July. I am noticing eggs and crawlers on the samples I have seen.

Top: Symptoms of pearleaf blister mites.  Bottom: Pearleaf blister eriophyid mites indicated by arrows.

We have received several pin oaks with vein pocket galls. A small female fly, called a midge, lays eggs on the leaf as it is just emerging from the bud and the larval feeding causes the tree to form the halls along the leaf veins and margins. The egg laying and insect feeding causes the leaves to produce the abnormal growth that we see along the veins. Luckily leaf galls are harmless to the plant. 

Pearleaf blister mites on a ornamental pear was an interesting sample. These mites is not a serious pest of ornamental pear trees and treatment options for homeowners are limited. In addition any treatment has to be applied well before the leaf symptoms are observes. 

General outdoor insects

Fungus gnats are common outdoors around homes right now.  Fungus gnat larvae feed on fungus and other decaying organic matter in moist areas. In homes one of the most common sources is over-watered house plants.

Velvet mites have been noticeable in gardens.  They look like tiny, bright-red moving dots.  I see them on my mulch and crawling across my landscape rocks.   Some red mites are common on sidewalks and are referred to as sidewalk mites or concrete mites.  Velvet mites are predators of other arthropods, so there is no reason for concern as these mites are beneficial. Here is a link with pictures:

We have received many inquiries about firefly larvae this year.  They look a bit like centipedes, but they will only have 6 legs.  Firefly larvae are beneficial as they eat pests such as slugs.   Also, for the second year in a row, we are hearing reports that this is a "good year for fireflies".  At least in some areas of the state the populations of these well-known, dusk-flying beetles are higher than normal.  That's good news and means moisture under plant debris on the ground, mild winter and other conditions were right last year for successful firefly breeding and survival.   Read more about fireflies at the Plant & Insect Diagnostic Clinic website


Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on June 23, 2017. The information contained within may not be the most current and accurate depending on when it is accessed.