Phytophthora Root Rot of Aleppo Pine Seedlings in a Forest Nursery in Spain

Since 1993, more than 120,000 ha of former agricultural lands has been forested in southern Spain. However, there is a lack of information about the etiology of the diseases affecting nursery seedlings in the region. During 1998 and 1999, death of Aleppo pine (Pinus halepensis) seedlings was associated with episodes of excess water. The plants were grown in containers in fertilized peat under shade cloth and remained waterlogged for long periods. A few weeks before their scheduled planting in the field, the plants exhibited needle wilting, extensive root necrosis, and root death. Phytophthora species were suspected as the cause of the mortality, and several species of the genus Phytophthora have been reported to cause damping-off of seedlings of Pinus spp. Only P. citrophthora, however, has been associated with Aleppo pine (1). In a nursery in southern Spain, P. drechsleri mating type A2 was isolated consistently from rotted roots of Pinus halepensis seedlings. Fungal identification was based on temperature-growth relationships and sporangial and gametangial characteristics (1). The optimum growth temperature was 24°C, with a rapid decline in growth rate above 30°C. Sporangia were persistent, terminal, nonpapillate, ovoid, and averaged 41 ± 4 × 29 ± 3 μm. The isolates were self-sterile, and antheridia and oogonia were only observed in pairings with A1 testers of P. drechsleri. Oogonia appeared smooth, spherical, and 33 ± 2 μm in diameter, with amphigynous antheridia that were 14 ± 1 μm long. Pathogenicity tests were conducted on 1-year-old seedlings of Pinus halepensis inoculated with two isolates of P. drechsleri collected from Aleppo pine and Quercus ilex seedlings, respectively, and two isolates of P. cinnamomi from Q. ilex. Inoculated and noninoculated control plants were waterlogged 2 days per week. After 18 weeks, both species of Phytophthora produced extensive root necrosis that resulted in needle wilting. Control plants did not develop needle symptoms and only had a low level of root necrosis. No difference in pathogenicity was observed between the isolates of either species. However, disease severity was less with isolates of P. drechsleri from Q. ilex than with those from Aleppo pine. To our knowledge, this is the first report of P. drechsleri causing root rot of Aleppo pine seedlings in forest tree nurseries in Spain and also demonstrates the potential for P. cinnamomi to cause damping-off on this pine species. Reference: (1) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996.