Disease

Sirococcus shoot blight caused by the fungus, Sirococcus conigenus (formerly Sirococcus strobilinus).

Importance

In the United States, Sirococcus shoot blight has become increasingly important since the early 1970's. When favorable conditions for infection have occurred, shoot and tree death have risen steadily in several local areas in the northern Lake States. Tree mortality in some Wisconsin red pine plantations, for example, increased from 19 to 67 percent from 1972 to 1976. Small seedlings and saplings growing under or near infected overstory trees are commonly killed by multiple infections (fig. 18). Trees not killed often became misshapen. Lower branches are often killed on pole-sized or larger trees as a result of multiple infections (fig. 19).

Figure 18	Figure 19

Biology

Initial infection occurs in juvenile needles and spreads into the stem to form a canker. A small drop of resin is often exuded at the base of the infected needle. The fungus spreads within the succulent stem tissue, but rarely into older wood. When infection occurs in the area of elongation, the restricted growth in the cankered area causes the shoot tip to curl over and form a crook (fig. 20). As the canker matures, small black fruiting bodies (pycnidia) form on dead needles under the fascicle and on shoots (fig. 21). These fruiting bodies produce large numbers of spores (conidia) that are spread to nearby red pine by rain splash. Under high humidity, mild temperatures, and low light, the spores germinate and infect red pine.

Figure 20	Figure 21

Figure 22

In the Lake States and the Northeastern States, major spore release and infection of current-year red and jack pine shoots occur in May and June. Symptoms such as pitch droplets, browning at the base of needles, chlorosis, shoot curling, shoot death, and needle droop (fig. 22) occur in late June, July, and August. The fungus overwinters in these dead shoots, and mature spores are produced and disseminated during rainy weather the following May and June, thus completing the life cycle.

Management Guidelines

The best control is to avoid two-story or uneven-age stands in northern areas where Sirococcus is a potentially serious problem.

• Remove infected overstory and understory trees (or infected shoots if understory trees are small enough). The removal of infected overstory trees removes a major inoculum source and the shade necessary for infection.

• Do not plant red pine seedlings under or adjacent to infected trees. A minimum buffer zone should be twice the height of the infected trees. This buffer zone can be left unplanted or it can be managed for deciduous species or immune or resistant conifers, such as white spruce, black spruce, balsam fir, or larch. As a precaution, newly established red pine plantations should be checked 5 years after planting to make sure the fungus has not spread into the plantations. If the fungus is present, carry out appropriate control measures to ensure that major inoculum sources are eliminated.

• Remove small pockets of infection in an otherwise healthy plantation by cutting infected overstory and understory trees. This procedure will minimize Sirococcus damage to the remaining healthy stand.

• Avoid microclimatic conditions favorable for infection in Sirococcus hazard areas. Humid, shady planting sites, such as small forest openings (diameter less than height of surrounding trees); shaded sites north and west of a stand of tall trees; and steep north or west slopes are potential hazard areas and should not be planted with red pine.

• Manage for resistant or immune conifers or decidu-ous species in high Sirococcus hazard areas where removal of scattered infected overstory red pine is impractical. Before choosing an alternate species for planting, make sure other important diseases and insects of the chosen species are not prevalent in the area.

Skilling, D.D.; O'Brien, J.T. 1973. How to identify Scleroderris canker and red pine shoot blight. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station. 6p.

Nicholls, T. H.; Robbins, K. 1984. Sirococcus shoot blight. For. Insect and Dis. Leafl. 166 Washington, DC: U.S. Department of Agriculture, Forest Service. 6 p.

Ostry, M.E.; Nicholls, TH. Biology and control of Sirococcus shoot blight on red pine. U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station. (In prep.).