Monterey pine forest made a remarkable recovery from pitch canker

Monterey pine (Pinus radiata) is a species of limited distribution, with three native populations in California. In 1986, a disease known as pitch canker, caused by the fungus Fusarium circinatum, was identified as the cause of extensive mortality in planted Monterey pines in Santa Cruz County. Monitoring studies on the Monterey Peninsula documented rapid progression of the disease in the native forest during the 1990s, with most trees sustaining some level of infection. However, between 1999 and 2013, the severity of pitch canker stabilized, with many previously diseased trees then free of symptoms, and plots monitored between 2011 and 2015 documented a steady decline in the occurrence of new infections. Consequently, whereas pitch canker was once a conspicuous visual blight in the forest, by the end of the observation period, symptomatic trees had become a rarity. The arrested development of pitch canker is suggestive of a reduction in the frequency and duration of fog near the coast, which provides conditions necessary for the pathogen to establish infections.

Jeffers’s Country

Chapter 1 opens with poet Robinson Jeffers’s introduction to the Big Sur landscape in 1914. Big Sur's rugged setting had long served as an obstacle to settlement or exploration, so that in the early century this coastline was sparsely populated and without modern technologies. Human endeavors had produced few permanent edifices, despite centuries of habitation and decades of small-scale extractive industries. The Spanish name for this coastline, “el sur,” represented how most people viewed the area in the eighteenth, nineteenth, and even into the twentieth centuries: as a rather inconsequential place that existed to the south of the more manageable—and profitable—Monterey Peninsula and its surrounding valley. Not until the 1920s, when highways and commercial tourism proceeded at a rapid pace throughout the country and Jeffers’s published verse on Big Sur gained popularity, did Big Sur’s isolation and underdevelopment become recast as a great asset. This chapter examines how Jeffers’s approbation of locals’ archaic mode of life helped to establish the sense that nature’s elemental forces and Big Sur’s inhabitants could together produce the most appealing landscape.

First Report of Grass Species (Poaceae) as Naturally Occurring Hosts of the Pine Pathogen Gibberella circinata

Gibberella circinata (anamorph Fusarium circinatum) causes pitch canker in pines and is not known to have any hosts outside the Pinaceae. However, G. circinata is closely related to and interfertile with G. subglutinans, which is associated with grasses both as a pathogen and a commensal endophyte. Furthermore, studies under controlled conditions have shown that G. circinata can colonize corn (Zea mays) without inducing symptoms (4). To determine if G. circinata can also infect grasses under natural conditions, plants were collected in proximity to trees with symptoms of pitch canker in native stands of Pinus radiata (Monterey pine) on the Monterey Peninsula and P. muricata (bishop pine) at Pt. Reyes National Seashore on the California coast during July and August of 2011. Leaves and stems were rinsed in 0.1% Tween 20, immersed in 70% ethanol for 30 s followed by 1 min in 1% NaOCl, and placed on a Fusarium selective medium (FSM) (1). Single-spore subcultures of colonies growing from cultured plant material were transferred to 0.6% KCl agar and identified as G. circinata based on morphological criteria as described by Gordon et al. (2). G. circinata isolates were recovered from Holcus lanatus and Festuca arundinacea on the Monterey Peninsula and H. lanatus at Pt. Reyes National Seashore. Three isolates from each of these sources (nine total) and one known G. circinata isolate from pines (GL 17) were tested for virulence by inoculating 1-year-old, greenhouse-grown Monterey pine trees; three trees were inoculated, once for each isolate. Trees were inoculated by depositing 250 spores in a wound on the main stem by the method described by Gordon et al. (3). Two weeks later, all grass isolates had induced resinous branch cankers with lesions comparable in length (17 to 24 mm) and appearance to those caused by GL 17. Similar results were obtained when inoculations were repeated. One isolate from F. arundinacea and one from H. lanatus (collected at Pt. Reyes National Seashore) were tested and shown to be somatically compatible with tester strains for vegetative compatibility groups C6 and C1, respectively, both of which are associated with isolates previously recovered from diseased pines (2). GL 17 and one isolate each from F. arundinaceae and H. lanatus were tested for their ability to infect F. arundinaceae cv. Fawn. For each isolate, 20 14-day-old seedlings (10 pots with two plants per pot) were sprayed to runoff with an aqueous suspension of 106 spores per ml. All inoculations were repeated once. Two weeks after inoculation, leaves and stems were rinsed briefly in 0.1% Tween 20, immersed for 10 s in 70% ethanol, followed by 30 s in 1% NaOCl, and cultured on FSM. All tested isolates were recovered from at least some of the inoculated plants (range 20 to 100%), from living stems and leaves, as well as from senescing tissue. These results show that grass species can be symptomless hosts for G. circinata, constituting the first documentation of any host for this pathogen outside the Pinaceae. Studies are underway to further characterize the host range of G. circinata and assess the epidemiological implications of grasses as alternate hosts for the pitch canker pathogen. References: (1) B. J. Aegerter and T. R. Gordon. For. Ecol. Manag. 235:14, 2006. (2) T. R. Gordon et al. Mycol. Res. 100:850, 1996. (3) T. R. Gordon et al. Hortscience 33:868, 1998. (4) C. L. Swett and T. R. Gordon. Phytopathology (Abstr.) 89:S126, 2009.