Impact of Seed Transmission on the Infection and Development of Pitch Canker Disease in Pinus radiata

The aim of this study was to determine the impact of seed inoculum on subsequent disease development in nurseries, specifically studying incidence of seed infection, transmission rate of seed to seedlings, and rate of disease development from the primary inoculum source. Transmission rate of Fusarium circinatum (F. circinatum) from symptomatic trees of Pinus radiata (P. radiata) to seed was 0.73%, being the fungus mainly on the coat. Seed infection incidence was positively correlated with tree disease severity. Seeds also become contaminated with F. circinatum during storage, where high relative humidity had a great effect regardless of temperature. Transmission rates from seeds to symptomatic seedlings measured at 48 days after seed inoculation with 10(4) and 10(6) was 28% and 80%, respectively. Seedlings showed symptoms of wilting and dieback, and eventually died within the total duration of this study (475 days). By this time, there were two remaining symptomless seedlings (1% of the total). F. circinatum was recovered from each of them, demonstrating its persistence in seedlings. Fungus populations in roots were estimated between 10(5) and 10(7) cfu/g in symptomatic plants for both inoculum doses and up to 10(4) cfu/g in asymptomatic plants. Disease infection rate was similar in post-emergence damping-off (up two months growing) and for established seedlings (up to eight months) after seeds were inoculated with 10(4) spores/mL.