Diseases and their Control

1

Diseases and their Control

Diseases can adversely affect Christmas tree plantations, mainly by weakening trees, reducing growth or making them unsightly. Fungi cause the majority of diseases in Christmas tree plantations. Fungi

reproduce by spores. They are spread by the wind, carried in raindrops, transported on equipment such as shearing knives or infested plant material.

Disease problems will be reduced by carefully selecting a proper site for the species being planted.

Proper planting and care can significantly reduce losses as vigorously growing trees are less susceptible to disease. It is important to provide adequate soil fertility; control insects; and avoid damage from herbicides, mowers and tractors. Practicing good sanitation techniques will reduce the occurrence and spread of diseases. Growers should remove dead branches, high stumps and any diseased/infected trees. However, even in well-managed plantations there can be outbreaks of infectious diseases. Early detection and control with appropriate treatments are essential. Using adapted and tested seed strains will help trees withstand many disease infestations.

The purpose of this section is to assist growers in identifying conifer diseases that can impact their plantation. The following diseases may be encountered in prairie Christmas tree plantations.

Disease Conifer affected Symptoms Brown spot

needle blight

Scots pine Yellow spots that turn reddish brown and eventually girdle the pine needles. These dark bands are surrounded by a yellow halo. Needle tissue beyond the band turns brown, producing a half needle scorch. Infected needles eventually turn brown and drop off tree.

Lirula needle blight

Spruce Yellow bands appear on needles in late summer/fall. The bands become light purplish brown and expand over the entire needle by the following spring. The needles then become reddish brown with a characteristic black basal band in the abscission layer at the base of the needle. Heavy infection results in the tree having a dirty green appearance Fir needle cast Balsam fir Discoloration and premature death of needles. The disease

seldom kills the tree, however severe infection reduces vigor of small trees and affects tree appearance.

Rhizosphaera needle cast

Spruce Needles turn purple to brown on current year’s growth;

usually begins at tree bottom. Dark brown to black fruiting bodies emerge through the stomata the spring after infection.

Stigmina needle cast

Spruce Similar symptoms to Rhizosphaera, can be differentiated by appearance of fuzzy fruiting bodies that emerge through stomata.

Western gall rust Pine This rust pathogen causes woody swellings (galls) and

cankers that deform branches and stems. It has no alternate

host and spreads directly from pine to pine. Christmas trees

lose their value due to stunting, witches broom and branch

death.

2 Brown Spot Needle Blight

1. Distribution and Disease Cycle

This fungal disease affects many pine species, attacking developing, susceptible needles as they emerge in summer. Spores released from fruiting bodies on infected needles are spread by rain, animals or human activities to healthy current season needles where they enter needles through stomatal pores.

Later in summer, new fruiting bodies are produced with mature spores that overwinter on infected or dead needles.

2. Symptoms and signs

Yellow, water-soaked looking spots form on infected needles, eventually becoming brown with yellow borders or bands. Lesions caused by brown spot needle blight do not usually appear until mid to late summer. Needles begin dying from the tips back until the needle is entirely brown, dropping in fall.

Infections often begin on lower branches on the north side of the tree. In minor infections, needles will drop but buds can usually produce new foliage the following season; in more severe infections, entire branches can be killed.

3. Control

If possible, remove and destroy infected branches, but do not prune trees during wet weather which can spread infection. Apply chlorothalonil as per label instructions beginning in spring when new shoot growth is 1-5 cm in length. Repeat applications at 3 to 4 week intervals during wet years or if infection is severe.

Brown spot infected needles of Scots pine. Photo courtesy Minnesota Department of Natural Resources, Bugwood.org

Typical brown spots on infected needles. Photo courtesy Edward Barnard Bugwood.org

Scots pine infected with brown spot. Photo courtesy Joseph O'Brien USDA Forest Service, Bugwood.org

3 Needle Cast Diseases

Rhizosphaera needle cast and stigmina needle cast are two of the most common diseases associated with spruce needle loss in the prairies. Needle diseases cause the most damage when the needles on the lower branches, the shady or wind-protected side and the interior crown stay wet for extended periods or when irrigation water contacts trees. If conditions favor disease development, the symptoms typically progress from the areas that are the most favorable to those that are less favorable during the course of several years. This may result in only the current-year needles remaining green, giving trees a sparse and hollow appearance.

Rhizosphaera needle cast and stigmina needle cast infect Colorado blue spruce and white spruce. Both diseases have similar signs and symptoms but can cause different amounts of damage and require somewhat different management strategies, making disease diagnosis essential. Both pathogens can be present at the same location and even on the same needle.

General Considerations for Needle Cast Diseases

The threshold of acceptable disease should be determined when evaluating whether disease management actions should be implemented. The threshold differs depending on the expected tree function. A Christmas tree is expected to maintain a full complement of green needles, while trees in a windbreak may achieve their function with substantial needle loss and some branch death. Needle loss that causes sustained growth reduction threatens the long-term health of a tree.

Typically, new shoots on a branch will have reduced growth if less than three age classes of green needles are present for two consecutive years. Trees also will have reduced growth of new shoots in the upper crown if more than about one-third of the lower crown has less than three age classes of green needles for two or three years. The impact on growth also depends on other factors, such as nutrient availability and stress.

Needle cast disease on Colorado spruce.

Photo courtesy USDA Forest Service, Bugwood.org

4 Rhizosphaera needle cast

1. Distribution and Disease Cycle

Rhizosphaera needlecast is a fungal disease caused by the fungus Rhizosphaera kalkhoffii affecting mainly Colorado spruce and occasionally other spruce. The disease begins in the lower portion of the tree with infected needles being shed, causing branches to look sparse. Spore dispersal from infected needles occurs during wet weather in spring, spreading by rain from needles infected the previous season to newly emerging needles. Under suitable conditions, the disease gradually progresses up the tree continuing to cause loss of needles, leading to eventual decline of trees.

2. Symptoms and Signs

Symptoms of rhizosphaera needlecast appear in the spring following infection, with infected inner (2nd year) needles turning yellow, then purplish-brown by end of summer, with black fruiting bodies

appearing in lines as they emerge from needles’ stomatal pores. Most infected needles will be shed by fall, although some may remain attached, acting as an infection source the following spring. Repeated infections will cause trees to begin having a sparse looking interior and after 3-4 years of severe infection, branches may begin to die.

3. Control

To reduce the spread of rhizosphaera needlecast, avoid pruning or shearing trees during wet weather and sterilize pruning tools frequently by dipping in 70% alcohol for 3 minutes. Remove any severely infected branches and rake fallen needles from the base of trees where practical. Promote good air circulation and encourage rapid drying of foliage by mowing weeds or other vegetation near trees.

Chemical controls registered for rhizosphaera needlecast control include fungicides containing chlorothalonil, propiconazole and trifloxystrobin. Apply as per label instructions in spring when new shoot growth is 1 to 5 cm in length, and again several times at 3-4 week intervals until conditions no longer favour disease development.

Infected needles with pycnidia (top) and healthy needle (bottom). Photo courtesy Michael Kangas, NDSU - North Dakota Forest Service, Bugwood.org

Rhizosphaera needle symptoms. Photo courtesy USDA Forest Service, Bugwood.org

5 Stigmina needle cast

Stigmina needle cast is associated with the fungus Stigmina lautii. This disease rarely was found in the prairies prior to 2006, but it now is known to be widespread east of the Rocky Mountains in the prairie provinces. Colorado and white spruce are highly susceptible.

1. Disease cycle

Stigmina lautii has a two-year life cycle. On most trees, fruiting bodies (sporocarps) develop by late spring the year after infection, mature that fall and produce spores that can cause infections the next spring. Spores typically develop in the spring just prior to new shoot growth. These spores can infect any needle age class throughout the growing season whenever temperatures are above 10

C.

2. Symptoms and Signs

Symptom development is similar to that of rhizosphaera needle cast. Infections result in narrow yellow bands and immature fruiting bodies that are not easily visible until the following year. Multiple bands and areas of fruiting bodies may develop on a single needle. Bands or entire needles may become discolored, ranging from yellow, purple, tan or reddish brown to brown. Discolored needles typically remain attached to the tree for an additional year following discoloration. The characteristic sign of this disease is the fruiting bodies that appear as small, black, fuzzy, round masses in the stomatal pores on all sides of infected needles. When mature, fruiting bodies are visible easily with a hand lens and appear as discontinuous black fuzzy lines.

3. Control

Cultural control is similar to that described for rhizosphaera needle cast. For Christmas tree plantations, needles should be protected for two months after bud break. Chemical controls registered for stigmina control include fungicides containing propiconazole and trifloxystrobin. Spraying should occur every year to ensure that four or more age classes of green needles are retained on trees.

Black fuzzy fruiting bodies emerging through the stomata. Photo courtesy Joseph O'Brien, USDA Forest Service, Bugwood.org

Stigmini fruiting bodies on green needles. Photo courtesy Joseph O'Brien, USDA Forest Service, Bugwood.org

6 Western Gall Rust

1. Distribution and Disease Cycle:

Current year’s pine shoots are infected when spores are dispersed in May and June from galls on branches and stems. Galls form in the summer following the year of infection, with sporulation not occurring until the subsequent spring. Galls can weaken stems, increasing the incidence of wind breakage. Extensive gall formation can cause tree stunting or mortality

2. Symptoms and signs:

Infection by western gall rust results in round or pear-shaped galls on branches or stems of susceptible pine hosts. In spring, the gall surface ruptures, releasing bright orange spores. Cankers sometimes form on main stems near galls; in some cases infection can result in production of witches’-brooms.

3. Control

Prune branch galls or remove trees with main stem galls and burn them. Avoid planting susceptible pine near diseased, natural stands. There are no chemical controls registered for Western gall rust.

P

O'Brien USDA Forest Service, Bugwood.org

Sporulating branch gall on Scots pine. Photo courtesy James Byler USDA Forest Service, Bugwood.org

Scots pine tree in a plantation showing symptoms of western gall rust. Photo courtesy William Jacobi Colorado State University, Bugwood.org