Have you ever noticed the fuzzy growth (Fig1) on the underside of an oak or sycamore leaf and wondered what was wrong with the tree? Fuzzy mats of hairy growth on the underside of tree leaves (Fig 2) are often mistaken for a plant disease or insect problem. In actuality, the whitish-tan fuzzy growth is a part of the plant known as trichomes (Fig3). Wikipedia defines a trichome as a small hair or other outgrowth from the epidermis of a plant, typically unicellular and glandular. Trichomes may provide greater surface area, and create a sunlight- or wind-deflecting blanket. Thick mats of trichomes on leaves can actually help a plant control its temperature. A carpet of fuzz on a leaf’s underside can reduce a plant’s water loss through evaporation. When viewed with magnification, trichomes can be seen to come in many forms including straight, branched, star-shaped, and tufted.
Oak leaf blister is caused by the fungus Taphrina caerulescens. Infections occur as buds swell and open during wet, spring conditions. Leaf blister symptoms usually appear within several weeks following infection as 1/4-1/2 inch circular, light green bulges on the top surface of leaves.(Fig 1) From the underside, the affected areas are sunken or depressed. These distortions may cause leaf bending or curling of narrow-leaved oak species. Some insect galls may resemble symptoms of oak leaf blister at first glance. (Fig 2) Upon closer inspection, the insect gall is a solid mass of leaf tissue as opposed to the distorted leaf blister caused by Taphrina. As the blisters age, they become dry, brown spots; severely diseased leaves may drop prematurely. (Fig 3.) Although this disease is quite conspicuous, it does not seriously harm healthy trees and control with fungicides is not usually recommended.
Volutella stem and leaf blight caused by the fungal pathogen Volutella pachysandricola can cause major damage to Pachysandra, destroying large areas in a bed. (Fig 1) Infected leaves first develop tan or brown blotches with dark brown margins, which expand, often with concentric lighter and darker target-like zones. (Figs 2a,b) Stem and stolon cankers appear as water-soaked diseased areas, turn brown, shrivel and often girdle the stem. causing stem dieback. (Figs 3a,b) Orangish spore masses develop in the cankered areas and the underside of infected leaves. (Fig 4) Volutella blight of pachysandra is often associated with plant stresses such as recent transplanting, exposure to bright sunlight, scale insects, and winter damage. Normally this disease does little damage to vigorous plants, thus providing good growing conditions is the most important control measure. Unfortunately, a dense planting bed of pachysandra is the desired horticultural outcome, thus thinning the planting to allow better[Read More…]
Bacterial leaf scorch (BLS) on oak is a systemic disease caused by the bacterium Xylella fastidiosa (Xf) (Fig 1). The bacteria live in the xylem vessels (water conducting elements) and restrict water flow. Xf is transmitted from tree to tree by xylem-feeding insects such as leafhoppers and treehoppers. Numerous woody hosts are susceptible to various strains of Xf . Symptoms of bacterial leaf scorch may vary somewhat between oak species. On pin oaks, scorching appears along the leaf margins and progresses inward toward the mid-vein (Fig 2). There is often a yellowish margin between the scorched leaf tissue and green tissue. On other red oaks, the scorch typically appears at the leaf tip and progresses up the leaf towards the petiole (Fig 3). Branches with leaves that appear to be healthy may be interspersed on the same tree amidst branches with scorched, diseased leaves (Fig 4). Leaf scorch and premature[Read More…]
Tubakia leaf spot, a fungal disease, infects all species of oak. However, oaks in the red oak group such as black, red and pin oak, appear to be most susceptible. Symptoms on oak include small to large dark brown or reddish-brown spots or blotches. (Figs 1,2,3) Spotting that occurs on leaf veins may cause large extended areas of dead leaf tissue along the veins (Fig 4). Marginal leaf scorch may also be seen to contain lesions of Tubakia (Fig 5). If trees are heavily infected with Tubakia leaf spot, premature defoliation may occur, however, the disease usually develops so late in the season that overall health of the tree is not affected. Fungicide sprays are not recommended. Cultural practices include maintaining good tree vigor by watering during drought stress periods and fertilizing trees appropriately. A type of anthracnose known as spot anthracnose, may also be present on leaves infected with[Read More…]
Boxwoods losing leaves should not be ignored!! Check them carefully for tell-tale symptoms of boxwood blight, a serious fungal disease that causes rapid defoliation and dieback (Fig. 1). The fungus that causes boxwood blight can infect all above ground portions of the shrub. The first symptoms of the disease are dark leaf spots (Fig. 2 ) that progress to twig blight and rapid defoliation (Fig. 3) and eventual death of the plant if it goes undetected (Fig 4). The narrow black streaks (cankers) that develop on green stems (Fig. 5), are a unique symptom that differentiates boxwood blight from other boxwood diseases. During periods of high humidity, white, fuzzy masses that consist of numerous clumps of spores will emerge from these black stem cankers (Fig. 6) as well as on the underside of leaf spots. The spores can be observed on infected stems and leaves with a hand lens. These[Read More…]
Getting calls from panicked customers about black spots on maple leaves? You’re probably not alone, because now is the time when people start to notice maple tar spot. Every summer we get questions about black spots on maple leaves that look like tar. These spots are not actually “tar” on maple, but are rather a fungal disease known as tar spot. These photos show a range of symptoms presented by this disease. Tar spots on maples are caused by fungi in the genus Rhytisma. The most common species are Rhytisma acerinum and R. punctatum. Symptoms first appear in late spring or early summer as infected leaves develop light green or yellow-green spots. During mid to late summer these produce black tar-like raised structures on the upper surface of leaves within the yellow spots. R. acerinum causes larger spots that are 0.5 to 2 cm in diameter; R.punctatum causes many small[Read More…]
One of the most common problems of broadleaf shade trees is a group of diseases collectively known as anthracnose. Anthracnose diseases are caused by fungi and become severe when cool, wet spring weather persists as leaves are first emerging. The most commonly affected trees are ash, white oak, maple, and sycamore. Dogwood, birch, elm, walnut, butternut, hickory, and other trees may also be damaged. Each species of tree is infected by a different species of fungus, thus the fungus does not spread from oak to maple or maple to ash or ash to sycamore. These fungi are referred to as host specific. While anthracnose diseases vary somewhat from one type of tree to another, they all cause death of leaf tissue and defoliation. Symptoms most often include irregular leaf spots and blotches (Figs 1,2,3, 4 ) The areas near veins are often most damaged and can lead to curled and[Read More…]
The Purdue Plant and Pest Diagnostic Lab (PPDL) recently received samples of Mugo Pine and Spruce that exhibited reddish-brown bands on needles of lower branches (Figs 1 & 2). Microscopic examination of needles confirmed the presence of Dothistroma,(Fig 3) a fungus that causes red band needle blight. Needles infected with Dothistroma first exhibit dark green bands on the needles that are quickly replaced with brown or reddish brown lesions. Only the base of the needle will remain green, with the remaining portion tan or brown. (Fig 4) Infection is typically more severe in the lower portion of the canopy nearest the ground. (Figs 5 & 6) In late Fall, black fruiting bodies (Fig 7) appear on needles and mature to release spores the following Spring and Summer. The spores are spread by wind and rain and can infect needles throughout the growing season. New needles are susceptible once they emerge[Read More…]
Three different rust fungi cause gall-like protrusions on eastern red cedar (Juniperus virginiana) trees. The bright orange gelatinous galls with tendril-like protrusions (Fig 1) resembling ‘koosh balls’ are caused by Gymnosporangium juniperi-virginianae. This fungal disease is known as Cedar Apple Rust. The galls develop on the cedar tree (Fig 2) and ‘sprout’ fungal spore-bearing tendrils during rainy periods in the spring (Fig 3 and 4). Spores from these galls infect apples and crabapples. The smaller, less conspicuous branch galls, with horn-like protrusions shown in Figures 5 and 6, are known as Hawthorn Rust and are caused by the fungus Gymnosporangium globosum. The fungus alternates between Juniperus spp. hosts; and hawthorn, crabapple, and apple in addition to several other rosaceous hosts. Gelatinous swellings on the cedar branches (Figs 7, 8, 9 and 10) are signs of yet another type of rust disease known as Quince Rust, caused by Gymnosporangium clavipes. This disease alternates between[Read More…]