Bright, orange-red lesions of Pear Trellis rust (Gymnosporangium sabinae) are still quite visible on ornamental pear trees right now. Take a look at the undersides of those rust lesions to find sporulating structures, called “aecia” (pronounced “ay-see-a”). These stringy, trellis-like structures release the aeciospores that will infect new shoots of juniper hosts (e.g. Juniperus sabina). Pear leaves require annual infection each spring from infected juniper hosts. Extended leaf wetness periods during leaf emergence, like the ones we had this spring, result in much greater levels of this rust disease on our beloved pear trees. The best course of action is to keep pear and juniper hosts as far away from each other as possible (e.g. a city block if possible).
With autumn mowing practices you have the opportunity for the cultural control of many ornamental plant diseases (and help discourage voles and mice from feeding on your trees). All those infected leaves dropping from the maples, oaks and crabapples are carrying fruiting structures that will be a source of disease next spring.
Research has shown that mowing/mulching fallen leaves regularly (e.g. weekly) and applying a light irrigation (and some water soluble nitrogen that is part of your fall fertilizer program) will help accelerate the breakdown of those diseased leaves over the autumn months. This means a significant reduction in the amount of fruiting structures that can start off the disease cycle next spring. Kevin Frank of Michigan State University suggests keeping mower blades sharp since leaves can be tougher than grass. Raise the mower height up and mow leaves when they are lightly wet (e.g. morning dew). This will keep the leaves from blowing all over and will prevent the mower from getting bogged down in wet leaves. The decomposing leaf litter is an excellent source of slow release nutrients and organic matter for the soil.
As leaves start to drop in container production, consider rounding up fallen leaves with a leaf vac/blower. Blow or Vacuum all the leaves to one part of the polyhouse for collection and removal. Similar to above, this cultural sanitation method can help reduce disease infections next spring.
Monitor for adults and eggs of spruce spider mite (Oligonychus ununguis) on conifers with a history of mite damage (Abies, Picea, Thuja). These are COOL season mites that will feed on current seasons’s foliage well into the fall. In fact, fall is a pretty important season for this mite species as they can feed way up until the end of November, depending on the temperatures.
A lot of horticulturalists don’t realize it but a lot of the damage from spruce spider mite feeding occurs in the fall, to be detected months later….in early spring. Infested plants have a slight grey-ish or dull cast to them from a distance.
Monitor lower branches, on the North and East side of the tree, this is where most of the spruce spider mite feeding damage is done. Shake branches over a flat white surface and look for tiny moving “specks”. You will need a hand lens to make out their light brown heads and legs and their black backs. You can also just harvest short twigs from this years growth from inside the canopy and examine them closely with your hand lens. Miticides (e.g. Floramite, Vendex) will be effective against both nymphs and adults where populations are very active. While you are monitoring, look for faster moving, pale tan coloured mites that could be feeding on the spruce spider mites. Predatory mites can sometimes help suppress populations of phytophagous mites, so keep an eye out for them when making pest management decisions.
Overwintering orange-brown eggs of spruce spider mite.