Tree and Shrub Disease Still a Problem in Northeast Ohio, Despite Dry Spell

It is a rarity over the past several wet years in Northeast Ohio, but we have finally hit something of a dry spell. Lawns are browning a bit and newly planted trees, shrubs and flowers are a touch weepy. Established plants are not threatened by this still short period of dry weather, but take special care with new plantings and give them a thorough watering.

Remember, with new trees and shrubs it is better to give them that deep thorough watering, then let the soil dry out to a depth of several inches before giving them another deep, thorough watering. This is better than a daily smattering of water from sprinkler irrigation. Use your brown-thumb skills – check to see if the soil is dry to a depth of a few inches before watering that new plant.

Over the past weeks we have gone from several inches above normal for our yearly precipitation to slightly below normal for the year. Therein lies a story. Since many infectious plant diseases caused by fungi and bacteria thrive on wet conditions, that should mean that we would not be seeing many disease problems in the landscape now, right? The answer is not that simple. We are still observing the results of that earlier wet weather. Here are a few examples:

GUIGNARDIA LEAF BLOTCH. The tell-tale brick-red to brown blotches are showing up on leaves of the common horse chestnut and its hybrids and our state tree, the Ohio buckeye. The Guignardia fungus got its start in those cool, wet days earlier this spring and now the harvest of those earlier infections is being reaped.

That's the trouble with diseases: the process of infection and spread of the infectious pathogen is hidden from our view. The fungal spores that infect during wet weather are microscopic in size and so unnoticed by our human eyes. They penetrate leaves and then mycelia of the fungus spread through leaf cells, still hidden from our view. Then, days and even weeks and sometimes months later the results of that hidden infection become evident as the plant develops symptoms of the host-parasite infection within.

For Guignardia blotch, those symptoms often end up with those brown patches spreading to encompass a good bit of the leaf, with yellow halos developing around the outside of the reddish-brown blotches. Overall quite an unsightly mess and probably the main reason that the otherwise quite lovely horse chestnut is rarely used as a street tree. After all, their spectacularly exotic flower panicles, palmate leaves, and stately tree form have a lot to offer.

What is the solution? Fungicides are an option, but would require repeated applications starting at leaf emergence in spring. More practically, consider the range of resistance within the genus Aesculus.

Aesculus is the genus of plants that includes horse chestnuts and buckeyes. Alas, Ohio buckeye (A. glabra), is quite susceptible to Guignardia blotch as is the common horse chestnut (A. hippocastanum). Yellow buckeye (A. flava) though is a large tree with moderate resistance and the tidy and red-flowered smaller red buckeye (A. pavia) has even better resistance. Finally the wonderful horizontal shrub, bottlebrush buckeye (A. parviflora) has almost no susceptibility to this disease.

Prevention is always better than a cure – and the best form of disease prevention for landscape plants is installing those species and cultivars with good genetic resistance to common diseases.

CRAB APPLE SCAB. This emphasis on prevention is indeed why we have disease evaluation trials at Ohio State University's Secrest Arboretum at the Ohio Agricultural Research and Development Center in Wooster. We recently did our first monthly apple scab evaluation of crab apples. As with many other diseases, infections by the scab fungus have probably not been frequent over the past several weeks, but the lag time between infection and symptoms means that we are seeing scab on many of the crab apples in our evaluation plot from earlier wet weather infections.

There is plenty of scab, but 31 of the 77 different crab apple types in the plot showed no scab in our ratings earlier this week. `Adirondack,' a white-flowered upright tree, `Strawberry Parfait,' an intense hot-pink-flowered erratic upright spreader, `Excalibur,' a golden-fruited rounded small tree, and `Louisa,' a soft-pink-flowered weeper are among the wide range of scab-resistant crab apples. Read all about it by checking out data from past years on the Web http://ohioline.osu.edu/sc195/027.html.

Why do we go to all this trouble? This letter from the Ohio Nursery and Landscape Association to the Ohio General Assembly says it well:

``Crab apple cultivars generate tens of millions of dollars in wholesale revenues for Ohio nurseries each year. This was not always the case, as important diseases resulted in reduced demand. Market demand for crab apples was revitalized by research at OARDC that identified disease resistant varieties. This research created a higher-value product for the consumer while decreasing production expenses for the grower.''

FIRE BLIGHT. Many landscapers and urban foresters have called this spring with questions about ornamental pears and another disease, bacterial fire blight. They are asking which cultivars of these pears have the best resistance to this disease, which has gotten worse in recent years. We know that `Aristocrat' and `Autumn Blaze' seem more susceptible, but answers are not as detailed as we can provide for scab on crab apples. That is why we have now decided to start a new project at Secrest Arboretum, evaluating ornamental pear germ plasm for fire blight and other problems (and for their positive attributes as well).

Enough disease talk. It has, after all, been relatively dry recently and so new infections for many diseases have slowed. In fact, let it rain! With that in mind, let's seed the clouds a bit with this designation of the tree of the week – soon to bloom in our area.

Tree of the week: goldenrain tree. Elizabeth Taylor. Montgomery Clift. The mysterious and magical plant in the woods. Raintree County. The legendary movie is not set in Ohio, but the golden yellow panicles of Koelreuteria paniculata (goldenrain tree) blooms are starting to grace Ohio landscapes now.

Goldenrain tree is a medium-sized tree (30 to 40 feet) with magnificent sunny-yellow flower panicles in late June and early to mid-July along the south-north axis in Ohio. It has an overall densely rounded form. Goldenrain tree has good tolerance of drought, alkaline soil pH and pollution conditions and does well as an urban tree where there is space for its growth. Bring on those goldenrain tree tresses and bring on the rain!

Author Jim Chatfield is a horticultural educator with Ohio State University Extension.