Needle cast diseases of conifer needles are caused by fungi. They can cause Christmas tree growers considerable losses when trees are tagged in the summer to be sold, only to be defoliated by fall.
Needle cast diseases are different from needle blights, like those caused by Botrytis on Fraser fir, as they primarily only affect the needles and not the shoots or stems. The typical symptoms produced by needle cast diseases include yellow or brown spots or bands on needles, needle tip discoloration or dieback, needle death, and premature shedding of needles. Sometimes affected needles can have a red or purple tinge. Typically, the affected needles are the previous year's needles. The current year's needles may look healthy, even though they too may be infected. Individual trees vary greatly in their genetic susceptibility to needle cast diseases.
Needle cast diseases are often difficult to diagnose. The symptoms are similar to low pH, poor nutrition, fertilizer or chemical burn, and even root rots.
Correct diagnosis of a needle cast requires microscopic examination of the size and shape of the spore produced by the fungus. Unfortunately, needle cast fungi produce spores infrequently. In fact, most needle cast diseases will infect needles one year, and not produce symptoms or additional spores until the following year. For many species of fungi, spores are only produced after needles have dropped from the trees.
Another common problem in diagnosing needle casts is that the pathogen may not be the only fungus present in diseased needles. As soon as the pathogen starts to kill the needle tissue, other fungi that are not true pathogens start to grow. These fungi are called saprophytes, since they live on dead tissue. Saprophytic fungi will also be present if the needles were killed by other means such as fertilizer burn or root rots.
The disease cycle. Most needle cast diseases have a one-year cycle. The fungi infect developing needles in the spring. Symptoms are expressed the following spring, and the spores are produced later in the season. Infected needles shed the year after they were first infected.
Of course, there are exceptions. Some needle cast diseases can cause symptoms on current year's needles. Others can produce symptoms on the previous two years of needles. In some instances, needle cast fungi grow down into the stem, causing death of the stem or branches. Defoliation over several seasons can also cause branch or even tree death.
The spores produced by the needle cast fungi eventually erupt through the needle, allowing spread of the disease. For many needle cast fungi, spores can still be produced after the needle has dropped to the ground. Correct diagnosis of a needle cast fungus is essential for control. This may require sending multiple samples over time to a diagnostic lab to catch the fungus when it is producing spores. Involving your county extension agent in this process is essential.
Most fungicides protect the plant from getting infected. They do not cure the plant after infection has occurred. Since fungi produce spores at different times of year, it is important to know which fungus is the true pathogen. Only in this way can fungicide applications be properly timed to be effective in controlling the disease.
Once a needle cast has been positively identified in your Christmas trees, there are four control strategies to prevent disease spread: 1) cut and burn problem trees, 2) increase air movement around trees so foliage does not stay wet as long, 3) maintain good fertility, and 4) apply accurately timed fungicide sprays. If the other three strategies are employed, fungicide treatments are typically not necessary.
Removing problem trees. Cutting and burning trees that have the worst symptoms can significantly reduce the number of spores available to spread and infect other trees. Usually only a few trees are initially diseased and would need to be culled. Remember that there may be trees which are infected but haven't shown symptoms. These may need to be culled the following year when symptoms are expressed. Problem trees are also most likely genetically predisposed to the needle cast disease, reducing the likelihood that other control measures will be effective.
Increasing air movement. Christmas tree growers may feel that premium trees must have heavy density foliage. Closer spacing of trees also increases profits per acre. However, most needle cast fungi require high humidity or even a film of water on the surface of the needle to infect the host. In western North Carolina, there tends to be more problems with needle cast diseases and needle blights in fields where the humidity remains high. Humidity in a field will increase when there are closely spaced, dense plants or poor weed control. Humidity is also higher in plantations near streams that are surrounded by hills or timber. The incidence of needle casts can often be reduced by modifying the site to increase air movement. This can be accomplished by harvesting plants, growing a more open tree, or not allowing weeds to grow too tall.
Proper fertility. One of the needle cast diseases of Fraser fir in western North Carolina, Rhizospharaea pini, is associated with trees grown on soils with low pH. Poor fertility is more of a problem than the disease. Take soil samples and plant tissue samples regularly, and fertilize according to the recommendations to avoid problems with needle cast diseases.
Proper fungicide timing. It is seldom necessary to treat for needle cast diseases in Christmas trees if the other cultural practices described above are followed. When fungicides are required, they should be applied when the needle cast is actively sporulating to protect neighboring plants from infection. Good coverage of current needles is essential for prevention of further infections.
Several needle casts have been identified in the foothills and mountains on different conifers grown for Christmas trees. They are listed below with a brief description. Remember that field diagnosis of needle casts is often very difficult. Accurate diagnosis is essential to properly time fungicide applications. For more information, contact your local Cooperative Extension agent.
Rhizosphaera kalkhoffii. A common needle cast in the mountains on spruces is Rhizosphaera kalkhoffii. Blue spruce is the most susceptible, though this pathogen can also cause problems in Virginia pine. Infection usually occurs in the spring, which is when fungicide applications should be applied. Symptoms can be seen in blue spruce only 3-4 months after needles first become infected. Infected needles may remain on the tree two years. Needles first turn yellow, and then purple-red. The fungus sporulates on the upper and lower surface of the needle through the stomates. Spores appear as rows of black dots along the needle. This disease is a worse problem in years with wet springs.
Rhizosphaera pini. Rhizosphaera pini needle cast was identified on Fraser fir in North Carolina in 1996. This fungus also produces spores through the stomates on the lower surface of the needle. Spores appear as black dots in rows on dead needles either on the tree or on the ground. This needle cast fungus is associated with trees growing in humid sites and at low soil pH where nutrition is poor. Currently, no fungicide recommendations are available. This fungus may function more as a saprophyte than a pathogen.
White pine needle cast diseases. In white pine, there are several potential needle cast diseases. The most pathogenic in the eastern US, Bifusella linearis and Canavirgella banfieldii, have both been identified in western North Carolina.
Many species of Lophodermium have also been reported as needle cast fungi. The majority are probably saprophytic. In the past, damage from needle cast fungi has been mistakenly attributed to ozone damage.
B. linearis and C. banfieldii commonly cause the tips of the current year's needles to die in early summer and turn tan, and finally reddish-brown by early fall. Not every needle will be affected in a group. The spores are produced and infect healthy trees in the spring at bud break. There are currently no fungicide recommendations for these fungi.
Brown spot needle blight caused by Mycosphaerella dearnessi occurs on many species of pine, especially Virginia and Scotch pine. The first symptoms appear in late August as yellow spots. Needle symptoms are usually found on the lower branches of the tree, especially on the north side. The needle tips turn brown, leaving for a time the base of the needle still green. Eventually, the entire needle dies and turns brown.
Cyclaneusma needle cast caused by Cyclaneusma minus produces symptoms on Scotch, mugo, and Virginia pines in fall and early winter. Fungal sporulating bodies when wet are creamy-white in color, which can help distinguish them from other needle diseases.
Publication date: April 24, 2014
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