Bacterial leaf scorch (BLS) is a devastating disease that has been shown to infect pin, bur, white, and shingle oaks in at least two areas of Illinois. The disease is discussed in issues no. 13 and 18 of this newsletter. One tidbit of information that was not mentioned in earlier issues is the theory that oak trees under drought stress show more damage from bacterial leaf scorch. If you are concerned about the threat of BLS to your oaks, consider supplying water regularly to your trees to encourage plant health. Research has shown that Virginia creeper plants infected with BLS showed more damage when under drought stress (article in Plant Disease, 85:1160-1164). This fact, along with field observations of the same tendency on oaks in Kentucky, suggests watering regularly may help oak tree health, which may help slow disease spread.
Foliar nematodes were confirmed in Illinois on hosta this season. This is not the first report for Illinois, but occurrences are still rather rare. These nematodes were discussed in issue no. 15. If you have hostas with symptoms of this nematode, try to get the problem confirmed and then remove plants if other hostas are nearby. The nematodes will live in the leaves until the foliage breaks down in the fall. As leaves drop and degrade, the nematode will move into the soil. At the very least, if you have suspect plants, remove old hosta foliage now, before the nematodes move into the soil to overwinter.
Phytoplasmas are not all bad. They are pathogens that can cause diseases such as ash yellows, aster yellows, and elm yellows (phloem necrosis). Phytoplasmas cannot be cultured on artificial media, making them more difficult to confirm in conventional labs. They can be extracted from the phloem tissues in which they reside and purified, and serological techniques have been developed for their identification. Such tests can be done by labs with capabilities to do these specialized techniques, but usually the cost is more than that of standard sample diagnosis. An interesting tidbit of information about phytoplasmas is that they are not always undesirable. Naturally grown poinsettias will become trees reaching 10 feet in height. Poinsettia growers strive to develop plants with a dwarfed, moderate branching growth habit for potted plants. Research initiated in 1995 has shown that the free-branching commercial poinsettia cultivars are actually infected with a phytoplasma that conveys these desirable traits. While free branching is actually a disease symptom in poinsettia, it is not harmful to the poinsettia and is very beneficial to the grower.