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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 Gibberella circinata (anamorph = Fusarium circinatum), causes canopy dieback and mortality in susceptible pine species in many parts of the world. Pitch canker is most problematic in areas with a relatively warm climate, suggesting a possible limitation on disease development imposed by low temperatures. To test this hypothesis, the effect of temperature on radial growth was examined in isolates of G. circinata of diverse geographic origin. All isolates grew most rapidly at 25°C and progressively more slowly at 20, 15, and 10°C. Spore germination occurred most rapidly at 20°C and was slowest at 10°C. To determine if the time required for spore germination might influence the likelihood of infection, the duration of wound susceptibility was examined by inoculating branches of susceptible Monterey pines (Pinus radiata). In each of six field trials, branches were wounded and then inoculated immediately or at 2, 6, or 9 days after wounding. The results indicated that wounds inoculated immediately became infected at a significantly higher rate than those inoculated 2 days later. Thus, if low temperatures extend the time required for germination beyond this period, a reduced infection frequency would be expected. Such a limiting effect of temperature could help to explain the current distribution of pitch canker.