Yard & Garden Line News Volume 3 Number 11                                                               July 15, 2001

Features this issue: And now...The Rust of the Story Coping With Dry Weather Premature Senescence and Leaf Shedding in Trees Entering the O(no!)zone For those of us who don't have too much zucchini… Sphinx Caterpillars in Gardens Editorial Notes

And now...The Rust of the Story
Jon F. Powell, Extension Turf Pathologist

Rust-infested turf. Photo: Dept. of Pl. Path.

From the mid-summer though fall, rust is a common disease of Kentucky bluegrass and perennial ryegrass. During severe epidemics on turfgrasses, the production of the orange spores (urediospores) by the rust fungus may cover shoes or clothing or may produce a orange cloud when the grass is mowed. Fortunately on turfgrasses, rusts are generally considered as minor pathogens but if not managed, they may contribute to poor aesthetics and lead to thinning of the stand.

Caused by a group of fungi identified as Puccinia, rusts have been important pathogens throughout mankinds history. Rusts were so destructive to grain crops that the Romans paid tribute to a rust god, Robigo. To appease the god the Romans made sacrifices of red dogs during a special holiday they called Robigalia. Rusts are still important diseases today, causing a loss of 1 million tons of wheat in North America alone, this number may increase 100 fold in years of severe epidemics.

Rust of turfgrasses occur in several forms ranging from crown rusts of perennial ryegrass to stem rust, stripe rust, and leaf rusts of Kentucky bluegrass. Even within each of these forms of rust, there are many forms of the rust fungi (races) that are specific to certain cultivars. In general, turf infected with rust will have a yellowish to reddish cast. Individual blades will appear chlorotic (light green to yellow) with yellow-orange or black spots. If you look closely at the yellow -orange areas with a magnifying glass you may notice the presence of numerous orange spores which are responsible for the spread of the disease.

Turfgrasses that are weakened due to excessively low mowing heights or are slow-growing due to low nitrogen fertility, shading, or lack of adequate moisture are most susceptible to rusts. Cultural recommendations to manage rusts begin with correcting these deficiencies. Perhaps the most important recommendation is to increase the nitrogen fertility supplied to the infected turf. However, care should be taken with fall fertility application so you do not promote the development of succulent tissues into winter dormancy which will predispose the turf to snow molds and winter injury. So, what level of fertility is sufficient? The goal should be to provide conditions in which the turf requires mowing on a weekly basis. By mowing the turf on a weekly basis you will be removing tissue that has the greatest potential to produce more spores and thereby reduce the potential for further infection. To promote overall health of the turf mowing heights should be increased to 2 inches. Under these conditions the turf will be able to tolerate rust infection.

Closeup of blades. Photo: Dept. of Plant Path.

Another time to keep rust management in mind is during establishment of the turf. Due to the limited amount of genetic variability within the improved cultivars of Kentucky bluegrass and perennial ryegrass, the improved cultivars are more susceptible to rust epidemics than are the common cultivars which are more genetically diverse. Additional information on which cultivars have greater resistance to rusts is available from the National Turf Evaluation Program (NTEP.org). As mentioned earlier, different races of the rust fungi are virulent on hosts with a specific genetic make-up. In a stand established as a single improved cultivar, if one plant is infected they all will be susceptible. Whereas if a stand is established from a common cultivar, only a small portion of the population will be susceptible to any race of the rust fungus. This is the main reason the common cultivars are less susceptible to rust epidemics than are improved cultivars. Fortunately, we can artificially create diversity by blending multiple improved cultivars to maintain the benefits of the improved cultivars (melting-out resistance, patch disease resistance, wear tolerance, etc...) and reduce the risk of severe rust epidemics.

Fungicides are another option for managing rust and can be used to provide complete disease management. The most effective products include the sterol inhibitors (propiconazole/Banner, fenarimol/Rubigan, triadimefon/Bayleton, and myclobutanil/Eagle) and the strobilurins (azoxystrobin/Heritage, trifloxystrobin/Compass, and pyraclostrobin/Insignia). The strobilurin fungicides have received considerable attention due to being labeled as "Reduced Risk" fungicides due in part to their low application rates and extended application intervals. Although the strobilurins have application intervals of up to a month and are used at very low rates, they are not recommended for use as curative treatments due to the risk of developing fungicide resistant strains of the rust.

Rusts have been a problem for mankind since the development of agriculture and will probably continue to do so for a long time to come. At least now we can manage rust by paying tribute to Robigo with an offering of Rubigan.

Coping With Dry Weather
Deborah Brown, Extension Horticulturist

Watering a garden. Photo: Deb Brown

Who'd have thought back in May or June that we'd be worrying about lack of moisture in mid-July? None-the-less, we've gotten into a hot, dry stretch (short-lived, hopefully) that is posing problems for lawns, gardens, and young woody landscape plants – all of them "spoiled" by the earlier cool, wet weather.

Whenever there's a prolonged hot, dry spell the subject of watering lawns and gardens looms large. Some municipalities have already imposed strict limitations on watering. But even where no restrictions exist, people wrestle with the idea of how often they must water to keep the plants in their landscape healthy and attractive.

Ideally, you should run your sprinklers or irrigation system long enough to wet the soil to a depth of five or six inches. However, hot, south-facing slopes, newly established lawns and lawns suffering from patch disease are exceptions. They need frequent, shallow watering when temperatures soar.

If you live in an area of sandy soil, you'll probably need to water lawns and gardens at least twice a week; three or more times in extreme heat. If your soil is loamy, thorough watering once a week should suffice in all but the hottest weather when you may have to water every four or five days.

Clay soil is a bit more problematic. Water usually puddles before sprinklers deliver sufficient quantity to encourage deep roots. Stop watering whenever the soil can't absorb more, then start again an hour or two later, until you've watered deeply enough.

When to water

Watering a lawn. Photo: Deb Brown

Cooler temperatures and calm conditions make early morning watering most efficient, but no harm is done to lawns and gardens by watering in the heat of the day. In fact, it helps cool heat-stressed plants. The problem is, much more water is lost to evaporation when you water in hot sunlight; less of it actually reaches the plants' roots.

Unfortunately, evening watering increases the risk of fungal diseases because foliage dries more slowly, if at all, at night. But the benefits of watering -- when plants really need the moisture -- outweigh any risks involved. A better choice, if possible, would be to water very early in the morning, say 4:30 or 5 am. Foliage will dry as the sun rises and begins to warm the plants.

Young trees

Moisture-stress is particularly harmful to the establishment and development of young trees that were planted this spring or have been in your landscape only a few years. Current recommendations are that you water them every couple days (perhaps even every day in extreme heat), using about a gallon and a half per inch of trunk diameter each time. Postpone watering if the soil is obviously still wet from the last watering – or if there's been good rainfall.

Mulching

Mulched tree. Photo: Deb Brown

Maintain about three inches of mulch in flower and vegetable gardens to reduce surface evaporation of water and eliminate the need for hoeing, which can damage plant roots and limit their ability to take in moisture. Grass clippings, straw, chipped leaves and shredded newspapers work well, and can be turned into the soil at the end of the growing season, along with some inexpensive lawn fertilizer (no "weed and feed" please) to help them break down.

Woodchips, shredded bark and cocoa bean hulls will accomplish the same thing, but are better suited for use around woody plants and perennials, where they needn't be disturbed for years.

Make sure there's a generous circle of mulch around young trees, starting just beyond their trunks. (You don't want to keep the trunks wet or bark can rot.) Not only will it conserve moisture, it will eliminate damage from weed whips (because there should be no need to use them by the trunks). Mulch will also substantially reduce competition from surrounding grass for needed moisture and nutrients.

Premature Senescence and Leaf Shedding in Trees
Patrick Weicherding, Extension Educator-Horticulture, Anoka County

Defoliating birch. Photo: Beth Jarvis

One of the many effects of drought (water) stress in trees is premature senescence and shedding of leaves. Given the hot, dry conditions we've experienced the past few weeks we can expect to see more of this happening, particularly in areas of the state that have not received adequate precipitation or in individual yards where trees have not received supplemental watering.

Leaf shedding under drought conditions may involve true abscission or leaves may simply wither and die. If dehydration occurs rapidly, due to exposure to dry, hot winds, leaves may wither and die but remain attached to the branch. Abscission of leaves injured by dehydration does not occur until the plants have adequate water, which suggests that resumption of metabolic activity is necessary for formation of the abscission layer. The actual physical process of leaf shedding under these conditions is associated with animals, wind or rain.

Under chronic drought conditions during the active part of the growing season, soil-water deficits cause abscisic acid to form in tree roots. As the abscisic acid is transported to leaves it inhibits bud and leaf development and promotes abscission of leaves. Leaf shedding under these conditions can occur without requiring adequate soil moisture.

The amount of premature senescence and leaf shedding varies with the season and among species. It is more common in older leaves; the youngest leaves often are retained after all the older leaves are shed. Injury to foliage and defoliation are also most apparent in portions of the crown that are in full sun. And, there are significant differences among species, with leaves of ring-porus hardwoods being most easily damaged (The term ring-porus means you can count the annual rings of wood easily). Some ring-porus trees include oak (Quercus), elm (Ulmus), hackberry (Celtis), hickory (Carya), walnut (Juglans), birch (Betula) and ash (Fraxinus). The notable exception is a diffuse-porus hardwood known as yellow or tulip poplar (Liriodendron). This species is notorious for shedding many leaves even during minor summer droughts.

Minor leaf shedding will not harm the tree. In fact, if water becomes available later in the growing season some trees defoliated by drought may produce a second crop of leaves from previously dormant buds. Many times these leaves will be smaller than normal but they will help the tree recover.

Entering the O(no!)zone
Janna Beckerman, Extension Plant Pathologist

Different species, even cultivars, have differing susceptibility to ozone damage. White pine is most sensitive to ozone damage. It's an indicator species. Silvering is characteristic of ozone damage. Photos: Dept. of Pl. Path.

The last week of June brought warm temperatures, high humidity and a somewhat unusual phenomenon, at least to Minnesota: Ozone. In the upper atmosphere, ozone (O3) actually serves as a beneficial UV filter. However, when it's produced at ground level due to the spontaneous interaction of heat and pollution, the effects on plants (and humans!) can be serious. Average ozone levels across the state are approximately 30 parts per billion (ppb). During the last week of June, we reached levels as high as 120 ppb!

Ozone is the probably the most common and most damaging form of air pollution. It is a highly reactive form of oxygen that causes a variety of symptoms. Foliar symptoms include flecking (silver or bleached-out spots), tissue death between the veins, stipple (numerous tiny spots of yellow, light tan, red-brown, dark brown, red, black, or purple pigment), mottling (irregular blotches of green, light green, and yellow), yellowing, bronzing, or bleaching. Plant growth may be stunted if foliar symptoms are severe. Flowering and bud formation can be depressed or aborted.

Of course, some plants are more susceptible than others. Conifers, particularly white (Pinus strobus) and jack (P. banksiana) pine frequently show a silvering of the foliage. Other conifers, such as red and Austrian pine, which are more resistant to ozone damage, may develop yellow to brown mottling and tipburn or a yellow to brown or orange-red flecking and banding of the needles. Ozone-killed tissues are readily infected by certain fungi, which may lead to the misdiagnosis of needlecast.

Broadleaf trees, shrubs and herbaceous plants develop different symptoms in response to ozone damage. Flecks and stippling occur in interveinal regions, while veins remain green. Mature leaves develop symptoms first, progressing to younger leaves. Young plants are generally more sensitive to ozone. Injured leaves may drop prematurely. The most sensitive broadleaf trees and shrubs include: Kentucky coffee tree, poplars, grape, Ohio buckeye, Prunus spp., cotoneaster and bridal-wreath, lilac, sumac and willow. Herbaceous plants that are extremely susceptible to ozone include pumpkins, watermelons, spinach and tomatoes

Upon diagnosis of ozone damage, we're confronted with the question of what to do? Every day, each of us inhales about 20,000 liters of air. Every time we breathe, we risk inhaling dangerous chemicals that have found their way into the air. Maybe our plants are trying to tell us something?

For those of us who don't have too much zucchini…
Janna Beckerman, Extension Plant Pathologist

Rhizopus soft rot on zucchini. Photo: Dept. of Pl. Path

As gardeners, we've all been accused of being that person who leaves zucchini on neighbor's doorsteps in the dead of night. Or perhaps you're the gardener who knows how to make zucchini bread, zucchini pancakes, zucchini relish, marinated zucchini, pickled zucchini and even zucchini a la mode. One rarely hears about disease problems with zucchini fruit. Heck, some gardeners WISH they had problems because then they could throw some away and not feel so guilty. This column is for those lucky gardeners who have blossom end rot and other postharvest disorders of zucchini and squash. Who says a little disease couldn't be a blessing in disguise?

There are two main rots that gardeners face: Rhizopus and Choanephora rot. Both soft rots are common pathogens of all cucurbits. As plant pathologists, its kind of unusual: Both Choanephora and Rhizopus are members of the Zygomycetidae (Zygomycetes, or 'zygos' for short). In fact, this entire group of fungi is given only six pages of consideration in Agrios' Plant Pathology textbook! Why? Well, only two genera of zygomycetes are considered plant pathogens (Have you guessed what they are?)!

Choanephora fruit rot may be considered the more aggressive of the two pathogens. It is a more serious pathogen of summer squash, especially during wet weather. Under these conditions, it can destroy both blossoms and young fruit. Invaded flowers turn brown and mushy and drop off the plant (Fig. 1). If fruit has begun to develop, the fungus invades and causes a wet rot. As the blossoms fade, they become covered with dense, white fungal growth. Eventually, the spores develop, turning this white mat of fungus black. It may appear as numerous little pins stuck in the fruit.

Rhizopus is the weaker of the two pathogens, and is considered a problem of wounded, overripened fruit. Rhizopus symptoms begin as water soaked lesions that enlarge quickly. Fruit collapses as it becomes colonized. If the fruit is broken, the fungus will be able to sporulate. These fruiting structures also resemble pins in a pincushion. This diseased tissue does not disintegrate, like it would if this were a bacterial soft rot.

Control of Rhizopus soft rot is relatively straightforward: Harvest zucchini promptly and avoid injury to the fruit. Unfortunately, there are no practical control measures to manage for Choanephora blossom rot. Fortunately (?), our weather pattern has changed to one of extreme wet (which favors Choanephora) to one of extreme drought, so the Choanephora may be taken care of for you. Besides, did you want all those zucchini anyways?

Test your diagnostic savvy monthly with Disease Watch at: http://www.extension.umn.edu/projects/yardandgarden/PlantPathWeb/Plpa.htm

Sphinx Caterpillars in Gardens
Jeff Hahn, Assistant Extension Entomologist

Large sphinx caterpillars (family Sphingidae) have been commonly seen recently. Also known as hornworms, they grow up to three to four inches long. They are smoothed skinned, usually green, and nearly always possessing a distinctive dorsal horn or tail on its posterior end, although in a few cases this may be reduced to a round bump. Many species have diagonal stripes while a few have spots or lateral stripes. Sphinx caterpillars feed on many plants, including garden flowers and vegetables and trees and shrubs.

Whitelined sphinx caterpillar Photo credit: Wayne Hansen U of M Extension Redwood Co.

The most common species being found is the whitelined sphinx moth. It is a bright green caterpillar with two black and yellow lateral stripes with a series of brownish spots. The stripes can also be broken and appear as a series of dashes. Whitelined sphinx caterpillars are generalist feeders and have been found on many different annuals and perennials. Normally, only one or a few caterpillars are seen at a time but there have been a few cases where large numbers were discovered. One home grape vineyard was found to be heavily infested with these caterpillars.

Physical removal is the most practical control of sphinx caterpillars in the garden, especially if there are only a few found. If larger numbers are discovered, consider how big they are. In nearly all cases by the time gardeners notice them, they are approaching full grown size and almost done with their feeding.

Get the low down on this month's insect pests at Insects http://www.extension.umn.edu/projects/yardandgarden/EntWeb/Ent.htm

Editorial Notes

Archie in his garden. Photo credit: Beth Jarvis

Good gardeners are clever and creative. Archie, at left, has been a Master Gardener in Hennepin Co. pretty much since the program started there. As you can see, he has raised beds in his yard, all equally spaced. Archie's quite a wood worker and he made a comfy seat for himself atop a board. The board has guides so Archie can scoot back and forth on sides of his raised beds. No more stooping!!

Next issue, Aug. 1, I'll be looking forward to Mary Meyer's article on ornamental grasses. I also hope to have the info on tetanus from the state health department. Mid- August, Bob Mugaas, Hennepin Co., has agreed to comment on fertilizers, whether organic or synthetic is truly superior plus other thoughts he may have on the subject.

This fall, Patrick Weicherding, Anoka Co., will share the latest research on winter injury/frost crack on trees. It may be more stress-related that we've ever dreamed. Sometime next winter, Doug Foulk, Ramsey-Washington, will talk about pears, what to know and how to grow. I'm also lining up articles on other plant breeding projects and will give you an inside look as they are available.

Please feel free to cut and paste any of the articles for use in your own newsletters. All we ask is that you give our authors credit.

Back issues Yard & Garden Line News are on the Yard & Garden Line home page at www.extension.umn.edu/yardandgarden/. Our home page has clickable links to most of the components of the Yard & Garden Line, such as Bell Museum of Natural History, INFO U and the Soil Testing Lab.

Deb Brown answers gardening questions on Minnesota Public Radio's (MPR) "Midmorning" program on the first Thursday of every month at 10 a.m. Katherine Lanpher hosts the program that is broadcast on KNOW 91.1 FM, and available state-wide on the MPR news radio stations.

If you would like to receive an e-mail reminder when the next issue of the Yard & Garden Line News is posted to the web, just send an e-mail to: listserv@lists.umn.edu (note: the second E in listserve is omitted), leave the subject line blank, then in the body of the message, type: sub yglnewslist
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Happy gardening!

Beth Jarvis
Yard & Garden Line Project Coordinator


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