Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
9/23/02
Contacts:
Paula Flynn, Plant Pathology, (515) 294-0581, pflynn@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for the Week Beginning Sept. 27, 2002
Plant Problems in 2002
By Paula Flynn
Plant Pathologist
Iowa State University Extension
Some plant diseases tend to be more of a nuisance than others in certain
years. The weather plays a big role in determining whether pathogens are
successful in invading plants. Some diseases are so well studied that
we can even predict their occurrence based on weather data with fairly
good accuracy.
Let's take a look at some of the common plant problems in 2002. The spring
weather was wet in a large portion of the state. Many fungi and bacteria
need extended wet periods to get into their host plant and cause damage.
You may have noticed shade trees such as ash, maple, sycamore and oak with brown spots on the leaves. Some of these trees, especially sycamore and ash, dropped many of the diseased leaves, creating a fall-like appearance. The common name for these springtime leaf diseases is anthracnose.
The most common anthracnose concern this year seemed to be on white oaks. Infected white oak leaves developed brown spots and brown edges and appeared scorched. Some tree owners worried that their trees had oak wilt, a serious disease of oak that can cause tree death. Although it creates an alarming, unsightly appearance, browning caused by anthracnose is of minor importance on established trees. Fungicide sprays are not necessary. As with most diseases removal of diseased leaves or plant parts is a good practice.
Rhizosphaera needle cast on spruce, especially blue spruce, is one of the most commonly submitted samples to Iowa State University's Plant Disease Clinic, 351 Bessey Hall, Ames, IA 50011, phone: 515-294-0581. Like anthracnose, the disease favors wet spring weather. As spring rains occur, the fungal spores spread the disease from sick needles to newly emerging needles. Over time, these infected needles turn purple-brown and fall from the tree prematurely. The disease takes its toll on trees over a period of years. Year after year of needle loss kills lower branches and works upward over time. If left uncontrolled, an infected tree begins to look like a Christmas tree sitting on top of a pole. Once infected, cultural practices alone won't control the disease. Fungicide sprays applied twice in the spring are needed to protect needles from infection. You can help prevent the disease by adequately spacing trees when planting to ensure good air circulation and also by planting several different tree species in your yard. Most diseases are fairly specific to certain types of trees, so diversifying your landscape slows down the damage caused by diseases.
As the 2002 season progressed, summer weather became hot and drought conditions were a problem in many counties. Diseases that interfere with the flow of water are really noticeable at these times. Dutch elm disease is a good example. This epidemic disease, which spread through Iowa towns from the 1950s through the 1970s, has continued to linger and threaten surviving American elms. Many elms lost the battle this summer. The symptoms of Dutch elm disease are easy to spot, even from a distance. Leaves in the upper canopy begin to wilt, turn yellow, then brown, and fall to the ground. If you remove the bark from these branches, you'll find dark brown streaks in the wood instead of a normal cream color. Even large trees infected with the fungus can lose their stately character in a matter of weeks. We still don't have an easy solution for saving our American elm trees. Removing diseased trees is important, because bark beetles can spread the fungus. Some owners chose to protect American elms from the disease with fungicide injections, a fairly costly practice. Work is continuing on developing Dutch elm disease tolerant or resistant elms, and some resistant elms are now available from nurseries and through catalogs.
As the summer came to a close, many people were surprised to find their shoes and pant legs orange after mowing their lawns. Rust on lawns has been common this season. Temperatures in the low 70s, high humidity and dew formation at night favor the disease. Rust-colored spots or pustules develop on grass blades. These pustules hold millions of fungal spores that help to move the fungus to new areas, including your shoes. Some lawns are more troubled by rust than others. Lawns that are under stress from drought conditions, soil compaction and low fertility tend to have the most problems with rust development. New seedings also are commonly troubled by rust. Fungicide sprays are usually not necessary. Be sure to use a blend of grass cultivars resistant to rust and other diseases when seeding new lawns areas. If possible, irrigate your lawn to reduce drought stress. Water early in the day rather than at night so the leaves dry off quickly. Also, be sure to use a balanced fertility program to keep grass vigorous. Finally, mowing frequently helps remove infected grass tips before the fungal spores are produced.
We'll have to wait until next spring to reflect on how the winter conditions affect the health of our plants. But there are a few things you can do this fall to reduce your chances of plant problems next year. If you're doing any planting this fall, be sure to select species that are hardy and adapted to your site conditions. If rainfall is limited, water plants deeply so they don't go into the winter under stress. Most importantly, remove any diseased leaves, stems, branches or other plant parts. Many plant pathogens have survival structures that allow them to lie in wait for the growth of the new, healthy tissue in the spring. So now is a good time to get the upper hand on common plant pathogens that might create problems in 2003.
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