Limiting Effects of Low Temperature on Growth and Spore Germination in Gibberella circinata, the Cause of Pitch Canker in Pine Species

Pitch canker, caused by the fungus Gibberella circinata (anamorph = Fusarium circinatum), was first identified in 1945 as a disease of pines in North Carolina (7). Pitch canker now occurs throughout the southeastern United States where it remains a chronic problem in plantations, nurseries, and seed orchards (2,3). In California, a pitch canker infestation that was first recognized in 1986 has expanded to affect 19 coastal counties (6). The most common symptom of pitch canker is dieback of branch tips resulting from girdling lesions. Severely affected trees also develop resinous cankers on large branches and the main stem, leading to extensive canopy dieback and death of the entire tree in some cases. In California, pitch canker is found primarily near the coast where moisture in the form of rain or fog coincides with moderate temperatures (6). Limited occurrence of the disease in cooler areas north of San Francisco (37°47′N) or montane locations farther inland cannot be explained by a lack of susceptible hosts. For example, many species common in the Sierra Nevada foothills are susceptible to pitch canker, including Pinus ponderosa, P. sabiniana, and P. coulteri (6). The pitch canker pathogen was discovered in the Sierra Nevada (El Dorado County) on Douglas-fir (Pseudotsuga menziesii) in 2003 (10), but the infestation appears to be isolated. Whether or not pitch canker becomes established in this area may depend on the extent to which environmental factors limit the ability of the pathogen to infect susceptible hosts. The current distribution of pitch canker in California suggests that cooler temperatures associated with northerly and montane environments may impose such a limitation. G. circinata requires a wound to establish an infection. Wounds may be caused by insects or injuries associated with weather or silvicultural practices (8,9). If germination and growth of the fungus proceed more slowly at cooler temperatures, wounded tissue may cease to be susceptible before the pathogen can establish an infection. The principal goal of this study was to characterize the limiting effects of low temperature on growth and spore germination in G. circinata in vitro. To determine if these effects of temperature could influence the pathogen’s ability to establish an infection under field conditions, an experiment was conducted to determine how long wounded tissue remains susceptible.