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Induced resistance to pitch canker, caused by asymptomatic Fusarium circinatum infection, in seedlings of Pinus radiata.
C. L. SWETT (1), T. R. Gordon (1). (1) University of California-Davis, Davis, CA, U.S.A.

<i>Fusarium circinatum</i> causes pitch canker, a destructive disease of Monterey pines (<i>Pinus radiata</i>) in native forests in California. Pitch canker is characterized by branch and stem cankers in trees of all age classes. Seedlings often die soon after they are infected, but some remain symptomless. The objectives of this study were to assess the frequency of symptomless infection in pine seedlings and to determine if this serves to induce resistance to subsequent challenge with the pathogen. Symptomless infection was examined in native seedlings and under greenhouse conditions, wherein seedlings encountered the pathogen in soil or as they emerged through an infested litter layer. In litter layer trials, <i>Fusarium circinatum</i> caused up to 55% mortality within eight months following emergence, and up to 60% of seedlings were asymptomatically infected. In native stands, <i>F. circinatum</i> was recovered from approximately 70% of seedlings, all of which appeared healthy. In challenge inoculations, seedlings grown in soil with low and high infestation levels sustained stem lesions 43% and 63% shorter than controls, respectively. Similarly, seedlings exposed to infested litter had a 10% lower infection frequency and mean lesion length was reduced by 28%, relative to controls. These findings suggest that induced resistance can reduce the impact of disease on seedlings. If so, seedling survival may not depend solely on disease escape, but could also involve adaptive defenses.<p><p>Keywords: Fungus, Trees, Conifers

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