Pathogen
The native fungal pathogen Ploioderma lethale causes Ploioderma needle cast of two- and three-needle pines in forests, managed landscapes, and Christmas tree farms (Sinclair and Lyon 2005).
Hosts
Based on samples submitted to the UMass Plant Diagnostic Laboratory, Ploioderma needle cast occurs most often on Austrian pine (Pinus nigra) and pitch pitch (P. rigida) in southern New England. Additional hosts include red pine (P. resinosa), Japanese black pine (P. thunbergii), and mugo pine (P. mugo). However, Ploioderma attacks several other hard pines in eastern North America, such as slash pine (P. elliottii), longleaf pine (P. palustris), and loblolly pine (P. taeda) (Sinclair and Lyon 2005). Interestingly, the pathogen is not known to occur on five-needle pines.
Symptoms & Signs
Ploioderma infects newly developing needles in early summer (June) in the northeast as new growth is developing (Sinclair and Lyon 2005). However, like many needle diseases of conifers, the symptoms do not appear for many months (approximately 6–10) after primary infection. In late winter to early spring, yellow lesions form on infected needles, and over time entire needle tips become brown to straw-colored while the needle base often remain green. Within the blighted needle tips, small, black-colored strips of fungal tissue (apothecia) are produced just below the epidermis. In late spring to early summer, the apothecia swell to become oval-shaped and rupture in the center to release large volumes of clear-colored spores (ascospores). The spores are spread by wind and splashing rainwater to initiate new infections on the current season's needles. Yet once again, symptoms on these infected needles will not appear until winter or late spring of the following year. Once the needles become symptomatic, they are typically shed by the end of the summer (Sinclair and Lyon 2005).
Management
Ploioderma needle cast is widespread in New England but is generally less damaging compared to other needle diseases of pine (e.g. Septorioides needle blight and brown spot needle blight). Typically, disease severity is highest on stressed Austrian pines and other susceptible pines grown as Christmas trees or dwarf cultivars with dense canopies. Like many needle cast diseases of pine, the damage is usually most severe on lower branches because of reduced air flow and higher humidity. Heavily infected branches should be pruned and removed from the site. The chronic loss of foliage can reduce vigor so keeping trees well-watered during extended dry periods is important, even though many hard pines are adapted to dry conditions. In addition, hard pines require full sun (≥8 hours per day in summer) and when planted in shaded settings or tight rows, they become more susceptible to serious needle cast outbreaks as they mature.
Fungicide applications may be warranted for high-value specimen trees, especially those predisposed by drought or insect feeding. Applications should occur when new shoots (candles) are 0.5-2" long and new needles are just emerging. Reapplications can be made at labeled intervals until needles are fully developed, depending on environmental conditions. Fungicides labeled for use against Ploioderma include: azoxystrobin, copper hydroxide, mancozeb, metconazole, and thiophanate-methyl.
References
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.