Above: Dark streaks in sapwood and affected leaves of an oak wilt infection. Photo by Gail Ruhl, Purdue University
Last week I covered insect pests of concern in Indiana, and today I’ll cover four plant diseases of concern in our landscapes and woodlands. Like last week, this is certainly not a comprehensive list of plant diseases; instead, it covers newer ones that have great potential to cause harm.
Thousand cankers disease of walnut (Geosmithia morbida) - When this disease was first recognized (in states other than Indiana) in 2008, we were understandably concerned here. With a sizeable population of walnut, it is among the most valuable hardwoods in Indiana. This disease is vectored (spread) by the walnut twig beetle, a tiny reddish-brown beetle about the size of a mustard seed. When it burrows into a tree to feed and reproduce, it carries the fungus with it, and tree tissue is infected. Tiny lesions (cankers) form and kill phloem tissue in the inner bark. Phloem tissue transports sugars to growing points in the tree. Thousands of beetles may attack a tree, and over time these tiny lesions merge and coalesce into larger areas that eventually choke this phloem tissue and kill the tree in 3-5 years. Mortality has been described as “death by a thousand cuts.” We have detected the beetle (without the fungus) and the fungus itself in separate, isolated areas in Indiana, but not in any standing walnut trees. For more information, see https://www.extension.purdue.edu/extmedia/BP/ID-478-W.pdf.
Boxwood blight (Calonectria pseudonaviculata) – This disease was first identified in Indiana in 2018. Symptoms begin as dark leaf spots that coalesce to form brown blotches. The undersides of infected leaves will show fuzzy white sporulation bodies following periods of high humidity. Boxwood blight causes rapid defoliation, usually starting on the lower branches and moving upward in the canopy. The result is usually the death of the plant. A key symptom that differentiates boxwood blight from other boxwood diseases is that narrow black streaks (cankers) develop on green stems. The fungus that causes boxwood blight can overwinter on infected plants and in infected leaf litter. American, English, and Korean boxwoods are all susceptible, along with Pachysandra (Japanese spurge) and Sarcococca (sweetbox). Fungicides may protect healthy boxwoods, but they won’t cure sick plants. For more information, see https://www.extension.purdue.edu/extmedia/bp/bp-203-w.pdf.
Sudden oak death (Phytophthora ramorum) - Sudden oak death is a plant pathogen that harms oaks in the red oak group (those with bristle-tips on the leaves) and other woody plants by causing bark cankers, leaf spots, twig dieback, and eventual death. Susceptible plants include oaks, rhododendrons, horse chestnuts, and laurels. Over 120 host species have been identified. There are 2 categories of hosts: those that host bark cankers, and those that host foliar symptoms. It was first found in Indiana on plants at retail outlets in 2019 when quarantine efforts were implemented. It spreads through infected plants, soil, and water so preventing its spread through timely diagnosis, disposal of contaminated material, and quarantines is paramount. For more information, see https://ag.purdue.edu/reportinvasive/species/sudden-oak-death/.
Oak wilt (Ceratocystis fagacearum) – this disease is fatal to oaks in the red and black oak group (those with bristle tips on the leaves) in Indiana and other Midwestern states. Trees such as white and bur oak appear to be more, but not totally resistant. The first symptoms appear in the top portions of trees. Leaves develop bronze and green tissue rather abruptly. Sapwood will show dark streaks in branches up to about 1 inch in diameter, inhibiting water movement in the tree. Leaf loss will occur. Infected trees will often produce a fungal mat beneath the bark the following spring. The disease is spread by sap-feeding beetles and root grafts between nearby trees. The highest risk for infection by beetles is from March 1 through the end of November. Beetles are attracted to fresh wounds, as would occur during pruning. So, pruning during the dormant season is recommended. If pruning must be done in the growing season, it is recommended that an appropriate tree wound dressing be used (one of the few instances when this is appropriate). Most all northern Indiana counties (including Whitley County) have had at least one confirmed case of oak wilt, along with many other Indiana counties. For more information, see https://www.extension.purdue.edu/extmedia/BP/BP-28-w.pdf.
See also Indiana DNR’s site on invasive species at https://www.in.gov/dnr/rules-and-regulations/invasive-species/.