Infection of Austrian, Scots and Ponderosa Pines by Diplodia pinea.
A study was made in eastern Nebraska on the season and conditions of dissemination and germination of D. pinea spores and of the susceptibility of needles to infection. Spores were disseminated from March to Nov., but large numbers were recorded only during periods of rainfall. Spores germinated on water agar between 12 and 36°C., germination % being highest at 24° and maximum growth of germ tubes occurring at 28°. Penetration of the needles took place through the stomata; direct penetration of the epidermis was not observed. Incubation of young shoots for 12 hr. at 100% r.h. was sufficient to cause infection on 2/3 of the shoots of Pinus nigra and P. ponderosa tested and on all the P. sylvestris. Inoculation of 10-year-old trees in the field indicated that young needles were most susceptible to infection from late April to mid-June; symptoms did not develop on the previous year's foliage. Results suggest that protection by fungicides is necessary during this period, and protection during periods of rainfall only is not enough, since 12 hours' exposure at high humidity may be sufficient for the infection of young shoots. KEYWORDS: Die back \ plant diseases \ fungal diseases \ Ceratocystis ulmi \ fungal diseases \ fungal diseases \ infection factors affecting \ fungal diseases \ infection from airborne spores \ fungal diseases \ infection through leaves \ fungal diseases \ foliage \ fungal diseases \ relation \ season \ Macrop