Meria needle cast, caused by Meria laricis, occurs throughout northeastern North America on larch (Larix spp.). The disease affects trees of all sizes, causing needle discoloration and death, leading to premature needle shedding. While infections typically cause only minor growth reduction on larger trees, younger trees in shaded settings may be severely defoliated leading to stunted growth and in combination with other stresses, death of the tree. Infections by Meria often predispose trees to damage by other insects, diseases and drought.
Meria needle cast affects all species of larch (Larix spp.), including eastern larch (L. laricina; also known as Tamarack), but is not known to infect any other conifers. While all species of larch are susceptible to infection, Asian species are less susceptible than European species and hybrids show intermediate susceptibility.
Symptoms & Disease Cycle
New infections are initiated in the spring when needles are elongating. Because larch is deciduous, severe infections can affect the entire canopy. Symptoms of infection appear suddenly in mid to late-spring, after prolonged wet weather and temperature between 50-77°F. The rapid onset of symptoms can sometimes be confused with frost damage but is unique in that the disease first appears as discrete lesions, while frost kills entire needles at once. Yellow to brown lesions are scattered across the surface of the needles. Small needle lesions then coalesce to encompass the entire needle surface, which leads to death and premature needle shedding. Lower portions of the canopy are more severely affected because of increased shade and moisture. During wet weather, the fungus produces clear spores in clusters that emerge from the stomata (used for gas exchange). These spore clusters are not visible with the naked eye but when viewed with a hand lens will appear as white dots which are slightly larger and more prominent than typical stomata. Spores are spread to adjacent branches and nearby trees by wind and rain. While Meria is capable of producing several generations of spores per year, successful infections are those that occur in the early season when needles are first emerging. In many cases, blighted needles do not abscise properly and remain in the canopy, serving as a primary source of inoculum in the future. The fungus also overwinters in blighted needles on the ground.
Always check planting stock for symptoms before transplanting to avoid introducing the disease to sites where it has not previously occurred. Establish new larch plantings in areas where larch has not been grown recently to prevent infections initiated from discarded needles on the ground. Thinning tightly grouped trees can improve airflow and boost vigor, which may minimize the effects of the disease. Chemical control of Meria needle cast should not be attempted in forest settings but may be economical for high-value trees. Fungicide application should begin just after bud break, when needles begin to expand and should continue at two to three week intervals until conditions are no longer suitable (dry, hot weather) or through July, whichever comes first. Chlorothalonil and propiconazole are labeled for use against the pathogen.
Written by: Susan Scheufele and Nicholas Brazee