Hot and Dry Summer Conditions

Issue: 
September-October 2005

SO FAR THIS YEAR in Wellington County, we have had nearly 30 days where temperatures reached or exceeded 29.5°C. That’s a lot of heat. Most areas were sitting well below 50% of normal precipitation accumulations until the rains came in late July.

Trees in the landscape don’t seem to be suffering as much as I thought they would. But that isn’t surprising. Since woody perennials (trees, shrubs, etc.) are slow-growing with a long life span, it can take 2-3 years before drought symptoms are expressed. And it can take another 2-3 years of good growing conditions before drought symptoms go away. So even though 2006 may have an excellent growing season with ample precipitation, don’t be surprised if you see lots of leaf scorch (marginal and interveinal necrosis) and dieback on deciduous trees.

On conifers, you may see needle tip browning, but it can be as bad as overall needle browning, dieback and mortality. Some of our exotic conifers (Norway spruce, Austrian pine) are often the first to show symptoms. Conifer mortality may become apparent in 2006, and if so, don’t forget how stressful 2005 was for these trees.

Attacking Cedar Leafminer
The moment you’ve all been waiting for has arrived. We are now in the spray window for systemic insecticides as a foliar treatment against cedar leafminer. Cedar leafminer larvae have hatched. They are very tiny and highly susceptible to foliar systemic insecticides such as di-methoate. By making an application in early-mid August, you will be targeting the proper insect stage. Another option is to sheer the foliage. This will remove infested tips and cut off the pest from the rest of the plant. The spring of 2005 saw significant damage to white cedar from this pest. Hopefully plants will be able to grow out of the damage through continued supplemental irrigation and a light fertilizer application at the end of September.

Another pest of white cedar, the spruce spider mite, has been sitting dormant all summer. This is a cool season mite that barely hatched out of the winter egg stage before it was hit with a 30°C heat wave at the beginning of June.

Spruce spider mites hardly had a chance to lay some eggs before they fell back into dormancy. The result is a relatively low population of summer eggs that will be hatching just as soon as the cooler temperatures arrive. Could this mean a reduction in spruce spider mite injury this fall? Maybe, but if we have a long, cool fall, the mite population could catch up pretty quickly.

Scale Crawlers
Be on the lookout for the second generation of pine needle scale crawlers. The second generation hatched at the beginning of August. Now is still a good time to catch the crawlers before they settle and develop a waxy covering (i.e. become nymphs). Pine needle scale crawlers are tiny, dark red specks that move around foliage. The adult stage is much easier to spot. They appear as white flecks that run along the length of the needles. Soon after that, magnolia scale crawlers will be hatching. This is one of the largest scale insects we have in Canada. At this time of year, the adults appear as large (approximately 8 mm long), salmon-coloured, convex bumps on the undersides of twigs. You can find them on most species of magnolia. Pick off the adult scales and you will see tiny, white grains underneath – those are the eggs. Crawlers are very tiny, crème-coloured and move over twigs and foliage (see photo). Monitor infested trees and treat at peak egg hatch.

Finally, Tar Spot Declines!
Due to extended periods of dry conditions during leaf emergence, tar spot on maples is not very prevalent this year. This is great news since tar spot is so unsightly and causes a lot of unnecessary concern with property owners. Tree health is not usually affected by this disease since the tar spots develop so late in the season. By then, leaves have already started to senesce and are not as functional as they were in early summer.

During the normal course of the disease, yellow halos on leaves are gradually filled in with small, black spots over the course of July and August. The black spots eventually fill in the yellow halo, giving the large spots a tar-like appearance. These black spots are the fruiting structures that will release infective spores the following spring during leaf emergence.

Since infection took place so long ago, fungicide applications are useless at this time. Removing fallen, infected leaves from susceptible trees is more effective if removal activities can take place in the entire neighbourhood, park or landscape.

Weekly mulching fallen leaves with the mower can help accelerate breakdown in autumn, leaving significantly less viable fruiting structures for the following spring.

Click here to view accompanying pdf tables for this article.

— Jen Llewellyn, Ontario Ministry of Agriculture and Food (OMAF) Nursery Crops Specialist

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