Forest Insect and Disease Newsletter
But like the story of Chicken Little, there may not be much to worry about.
In many parts of the state we experienced prolonged periods of cool wet weather during leaf expansion. It is likely to be a good year for the development of anthracnose on hardwoods such as ash, maple, and oak. Anthracnose is a general term for a disease caused by dozens of species of host-specific fungi. These fungi infect leaves via airborne spores, invade epidermal cells, and cause necrotic lesions that expand on leaf tissue during the growing season. If defoliation occurs early enough in the spring, trees will leaf out again, and the overall health of the tree is generally not affected.
Some tree species, such as ash and maple, will shed infected leaves prematurely in response to anthracnose infection. Ash leaflets develop irregular brown blotches, dead tips, dead areas along leaf veins and may eventually fall off altogether. Leaves that are retained will often curl, turn brown, and make the tree unsightly if severely affected.
Oaks may also be infected by anthracnose during cool, wet conditions, with white oaks being the most susceptible. Leaf blotches may develop between leaf veins, eventually becoming tan and papery. Leaves may also be curled or shriveled. When leaves near maturity, the leaves become more resistant to the pathogen so the size of the lesions decreases.
To manage anthracnose, maintain tree health by watering regularly during dry periods, mulching properly, raking up dead leaves, and pruning out dead branches and twigs. Fungicides are generally not recommended. Raking and destroying infected leaves in the fall will reduce disease inoculum for the following year.
There are three common needle diseases of spruce trees in Minnesota: Chrysomyxa needle rust, Rhizosphaera needlecast and Lirula needlecast. Colorado blue spruce, white spruce and black spruce are the most widely- distributed species; white and black spruces are native to Minnesota. Blue spruce has been an increasingly popular species to "spruce up" yards and landscape and, in fact, they are handed out as door prizes to school kids as well as being popular with landscape companies. After years of having to prescribe disease control and treatment schemes to landowners to remedy their needle disease challenges, I question the advisability of planting blue spruce. Let's look at three, more common varieties of needle rusts and casts that can be seen on the needles of spruce trees in early summer.
1. Chrysomyxa needle rust
During the summer of 2011, spruce needle rust was common everywhere I traveled. The first time I saw this disease was in the late 1970's while I was on a field trip back from Beltrami Island along one of the trails that run from the Pitt Grade to Faunce Rd. A young stand of white spruce glistened orange like it was someone's yard being decorated for Christmas. I had never seen that before. When I came back to the office I did some research of what literature was available at the time. Not much information was available except in some Canadian publications.
This rust is caused by a fungus species, Chrysomyxa ledicola and is common on white, black, and Colorado blue spruce, among others. It infects the current year's needles during cool, wet, spring weather and causes them to discolor later in the season. Interestingly, the yellowing is due to the addition of carotene pigments from the fungus as well as the breakdown of chlorophyll in the needle. At the same time, they develop fruiting structures that produce spores which infect Labrador tea, the alternate host of the fungus. Needles eventually turn brown and fall off.
In most cases, spruce needle rust is a cosmetic problem and control is not needed. Here are some ideas for managing spruce needle rust:
- Reduce the moisture level during early spring and summer by proper tree spacing, allowing better air flow and moisture evaporation. Direct your water sprayer so it does not hit the branches (although that may be hard to do if the branches reach the ground).
- Avoid planting susceptible tree species.
2. Rhizosphaera needlecast
Colorado blue spruce is a popular tree and certainly beautifies a yard. I have some in my yard that were transplanted twenty years ago. Every year I plan to cut them down for pitchy firewood. They are tall and spindly with virtually no branches on the bottom 70 percent of the crown. They are alive and that's about it. Almost every blue spruce I have been asked about and nearly all that I haven't been asked about have more dead needles than live needles. And it's mainly due to Rhizosphaera needlecast.
The disease is caused by the fungus R. kalkhoffi. The fungus attacks needles on the lower branches first, then moves upward. Eventually, if the tree lives long enough, the entire crown will look the same. The one-year-old needles become infected in May and early June, turning yellowish to purple-brown. Next spring these needles will shed, or cast. By examining the dead needles with a hand lens, you can see rows of tiny black dots (fruiting bodies) that are growing out of the needle's stomatal openings. The spores are rain-splashed and can be wind-blown during rain to nearby spruce.
Severely infected trees may have green branch tips (this year's new needles) with all inner branch needles discolored, dead or missing. Eventually, when enough needles die and drop off, the branch dies. In a few years you will end up pruning off those dead branches for cosmetic and safety reasons.
The best management option available is to plant another, less susceptible species. Other options include:
- Most importantly, try to buy planting stock that is free of symptoms and doesn't have fruiting bodies on the needles. Black Hills spruce is a much less susceptible species.
- Remove severely infected trees, as they are reservoirs of infection.
- Don't plant new trees close to infected trees. The stress of being newly planted can make them susceptible to infections.
- Reduce stress by watering trees during drought periods. But don't allow the spray to directly hit the needles, as this will aid the growth of infective spores.
- Don't shear susceptible trees. This promotes increased twig and needle growth at the ends of branches and increases humidity inside the crown, which favors disease development.
- A fungicidal program can work, but must be applied twice a year for at least two consecutive growing seasons to be effective.
Lirula needlecast, caused by Lirula macrocarpa, is found on blue, white, Black Hills, Engelmann, and black spruce. Colorado blue and Black Hills spruce are the most commonly and severely affected. I remember seeing some branches with second-year needles appearing tan with characteristic black lines on the needle surface long before I could determine what it was.
The fungus infects the current year's needles, which become tan to light brown by the spring of the second year. During the third year, raised black lines form on the needle surface along the mid-rib of the needle. These are the fruiting bodies. In some cases horizontal black lines also form across the needle; these are fungal parasites of Lirula working inside the needle. In late spring and early summer, rain-splashed spores are released. As with other needlecasts and rusts, needles on the lower branches are more severely infected. Infected needles turn grayish-brown to tan but remain attached for several years and serve as a source of infection. It takes three to four years for Lirula to complete its life cycle in northern Minnesota.
- Avoid planting new spruce trees near infected trees.
- When purchasing plant stock, examine the second year needles carefully before purchasing a new problem.
- If you have heavily infected trees it might be best to remove and destroy them before attempting chemical controls.
- Apply fungicides when needles are about half the length they would be if mature. Be sure to follow label directions to determine if follow-up is necessary. In addition,fungicide would need to be applied for at least three years, since the life cycle lasts three years.
For more information on other spruce insects and diseases, please see the Spruce diagnostic sheets.