Effect of Dissolved Oxygen Concentration on the Relative Susceptibility of Shortleaf and Loblolly Pine Root Tips to Phytophthora cinnamomi

Fraedrich, S. W., and Tainter, F. H. 1989. Effect of dissolved oxygen concentration on the relative susceptibility of shortleaf and loblolly pine root tips to Phytophthora cinnamomi. Phytopathology 79:1114-1118. Exposure of shortleaf and loblolly pine lateral roots to oxygen tion had no subsequent effect on root susceptibility to infection or the concentrations of 0-0.25 mg/ L for durations of 6-24 hr before inoculation rate of symptom development in infected roots of either pine species. increased root susceptibility to infection by Phytophthora cinnamomi Results suggest that essentially anaerobic conditions are necessary in the as compared with roots maintained continuously under oxygen concenrhizosphere before pine root tips are predisposed to attack by P. cinnatrations of 7 mg/ L. Symptom development in lateral roots progressed momi. Further, no evidence was found to support the hypothesis that more rapidly in roots that had been stressed with low oxygen than in differences in tolerance to low soil oxygen concentrations between loblolly roots maintained under high oxygen. Maintenance of pine seedlings at and shortleaf pine are linked to their relative susceptibilities to infection oxygen concentrations of 0.25-1.0 mg/ L for 12 and 24 hr before inoculaby P. cinnamomi. Plant species are known to vary in their tolerance to soil Kuan and Erwin (20) found that mortality of alfalfa (Medicago saturation and the low oxygen environment that results under sativa) seedlings due to root rot caused by P. megasperma this condition (16,17,27). Although definitive comparative studies increased as the number of days of preinoculation soil saturation have not been conducted on the tolerance of loblolly pine (Pinus increased. Likewise, cultivars of Rhododendron spp., which were taeda L.) and shortleaf (Pinus echinata Mill.) pine to low soil otherwise resistant to Phytophthora root rot, developed severe oxygen concentrations, several authorities have considered disease symptoms when root systems were flooded for periods loblolly to be the more tolerant of the two species (15,16,28). of 48 hr before inoculation with P. cinnamomi (1). Loblolly pine is rated as a moderately flood tolerant species, Low oxygen conditions in the rhizosphere, which have been whereas shortleaf pine is considered to be among the least tolerant induced with nitrogen gas before inoculation, have predisposed of the forest tree species (16). Where the range of these two species roots to infection by fungi. Bean (Phaseolus vulgaris) plants overlap, their ecological niches tend to be complimentary, with exposed to low soil oxygen conditions for a period of 3 days loblolly pine favored by the heavier-textured, wetter soils, and were predisposed to attack by Fusarium solani f. sp. pisi, which shortleaf pine by those sites that are lighter and drier (15,23). otherwise is nonpathogenic on bean (22). In studies of root and Low oxygen regimes affect both pine species, although Zak (28) crown rot of Mahaleb cherry (Prunus mahaleb) seedlings caused concluded that loblolly pine exhibited greater tolerance to poor by P. cryptogea, reduction of soil atmosphere to 0.5% oxygen soil aeration than did shortleaf pine. for a 24-hr period after inoculation greatly increased disease Soil types upon which littleleaf disease of pine'has been most severity as compared with seedlings maintained in soil severe typically have poor internal drainage (3,4) and can have atmospheres of 2.5 and 21% oxygen (26). water tables that are perched for extended periods during the The purpose of our study was to determine if exposure of winter and spring months, particularly in years of high rainfall loblolly and shortleaf pine roots to low oxygen concentrations (13,14). In addition, the subsoil consistence of high hazard sites for varied time periods before inoculation could alter root tends to be firm and plastic, and mottling typical of low aeration susceptibility to infection by P. cinnamomi and, if applicable, is present in some soils (3,14). The importance of poor internal to determine if responses to low oxygen differed for the two pine drainage in the development of littleleaf disease has been assumed species. to be related to the production and dissemination of zoospores by Phytophthora cinnamomi Rands (2,28). Soil oxygen concentration is regulated by all factors that affect MATERIALS AND METHODS diffusion coefficients, and by all factors that affect soil respiratory activity. Primary factors affecting the diffusion coefficient of Seedling and zoospore production. Loblolly and shortleaf pine oxygen into soil are soil moisture content and soil structure (11). seedlings were grown from seed in sand culture as previously Oxygen has a slow diffusion rate into, and a low solubility in, described (10). Seed were obtained from the International Forest saturated soils and, therefore, oxygen may be depleted in sites Seed Co., Birmingham, AL. Plants were grown in growth of high metabolic activity (8,9,12,25). Oxygen levels in soil can chambers at 24 ±_ 1 C with a 16-hr photoperiod for 6-7 wk. decrease from 21 to 0% by volume in less than 24 hr if oxygen Light intensity in growth chambers was approximately 1,750 ±_ consumption is high (21). In addition, due to the heterogeneous 50 lx. nature of soil, some anoxic microsites may occur in soil even Zoospore inoculum was produced as described previously (10), though the average oxygen concentration may be close to atmosand quantified and handled as follows. Upon release of zoospores pheric levels (11). from sporangia, spores were poured through four layers of sterile The varying degrees of oxygen deficiency experienced by roots cheesecloth, mixed gently, and five 10-ml samples were in saturated soils can cause physiologic damage resulting from immediately withdrawn and placed in test tubes. Zoospores in both the direct and indirect effects of low oxygen concentrations, stock solutions were held at 18 C until used for inoculations. adthis damage may predispose roots to infection (8,9,19,25). Test tubes containing zoospores were placed on a vortex mixer and for 30 sec, and 10 counts of cysts were made for each sample with the aid of a hemacytometer. Mean zoospore concentration © 1989 The American Phytopathological Society of each sample was determined, and an overall mean concentration