Department of Plant Pathology

The recent phase-out of the soil fumigant methyl bromide (MB) due to its impact on stratospheric ozone pre-sents a huge challenge to strawberry nursery producers. We evaluated the effectiveness of alternative fumigants on soil pests and plant productivity, as well as production costs in California strawberry nurseries. Our trials followed nursery stock through low- and high-elevation phases of runner-plant propagation and a complete cycle of fruit production in coastal fields. Plant yields from the nurseries and fruit yields from Oxnard and Watsonville indicated that nursery plots treated with iodomethane plus chloropicrin, with 1,3-dichloropropene followed by dazomet, and with chloropicrin followed by dazomet produced runner-plant yields that were similar to methyl bromide plus chloropicrin. However, our economic analysis suggests that nursery profitability may nonetheless suffer from the loss of methyl bromide.

We developed and tested an integrated pest management (IPM) program for the key pests of cut roses, which was based on fixed precision sampling plans, thresholds, biological control, directed sprays of reduced-risk pesticides, and cultural control. This program represented the largest effort to date to implement an IPM program in U.S. floriculture. The biological control of mites was successful at all locations, and pesticide use was generally lower in the IPM greenhouses. Future work will concentrate on reducing scouting time, improving natural-enemy release methods, and developing IPM techniques for secondary pests and powdery mildew.

While walnut trees on Juglans hindsii x J. regia ‘Paradox’ rootstocks are highly susceptible to crown gall, it is unknown whether this bacterial disease is acquired in the nursery or the orchard. We selected two groups of gall-free trees in nurseries, those adjacent to trees with and without galls. Two years after being transplanted in the orchard, trees in the group adjacent to those with galls had significantly greater — more than four times more — crown gall incidence than those adjacent to trees without galls (14% versus 3%). In addition, trees in prolonged (17-day), bare-root, unrefrigerated storage before transplanting were associated with higher crown-gall incidence. We also found that crown gall can decrease walnut tree productivity. For every quarter of trunk circumference that was galled, there was a 12% decrease in cumulative nut yield over the first 4 years of production.

In recent years, lenticel rot of potato tubers, caused by the bacterium Erwinia carotovora subsp. carotovora, has become an economically important postharvest disease for Kern County growers. Disease symptoms are sunken and rotted tissue surrounding tuber lenticels, which develop after harvest and packing. In the field, the bacterium also causes Erwinia early dying, characterized by wilt and progressive necrosis of leaves, eventually resulting in potato plant death. This study confirms Erwinia carotovora subsp. carotovora as the causal agent of both problems in Kern County and establishes the link between the field and postharvest diseases. Control of both diseases is difficult and relies on the integration of cultural methods, from preplant seed-piece handling to postharvest processing.

Dieback, or “dead arm,” in noncoastal California grapevines is most commonly caused by Botryosphaeria spp. Using Koch’s postulates, we demonstrated that isolates of B. obtusa are pathogenic on grapevines. We initiated studies to investigate the life cycle of B. obtusa and ways to control it with cultural practices. Fungal spores disseminated by rainstorms were collected in traps in an Arbuckle vineyard from December 2006 through spring 2007. The data suggests that B. obtusa was rain-disseminated throughout winter and spring, and that pycnidia on deadwood in the vines is a major source of inoculum for new infections. Transmission may also be possible via vegetative propagation, pruning shears and insects. Durable latex paints were investigated for protecting pruning and surgical wounds; a self-priming latex paint was shown to be an effective barrier and was nonphytotoxic.

A key strategy for biofuel production is making use of the chemical energy stored in plant cell walls. Cell walls are a strong meshwork of sugar chains and other polymers that encircle each plant cell. Collectively known as lignocellulose, cell wall material represents the bulk of plant dry mass. Biofuels can be made by releasing sugars from lignocellulose and converting them into fuel; however, this is currently an energy-intensive process. We summarize the barriers to efficient lignocellulosic biofuel production and highlight scientific research recently funded by the U.S. Department of Agriculture and U.S. Department of Energy, both to understand and harness the mechanisms by which plants build cell walls, and to further develop enzymes and microbes that facilitate sugar release and biofuel production.

Early detection and rapid response are crucial in any effort to reduce the risk of new and emerging biological threats to crops and other plant resources. This underscores the importance of having the necessary diagnostic expertise, infrastructure and resources in place. Three programs — the National Plant Diagnostic Network, the National Clean Plant Network and the Citrus Clonal Protection Program — illustrate how accurate and rapid diagnosis plays a critical role in providing healthy plants for growers and in securing production systems for food and fiber. These three programs depend on statewide, regional and national networking among university, state and federal scientists, regulatory officials and industry members to help mitigate the impacts of plant pests and diseases.

Viruses are of great concern in vineyards. They cost the California wine grape industry as much as $91,661 per acre over the life of a vineyard, according to a 2015 economic study of the North Coast wine-growing region. As a first step toward managing viruses, growers are encouraged to plant certified material regulated by the California Grapevine Registration and Certification program. There are risks in sourcing plant material from stocks that are not subject to the same level of regulation. We surveyed vineyards of varying ages for eight common viruses to demonstrate the value of selecting certified material for new plantings.

As the information revolution sweeps through the agricultural sector, extension professionals may be lagging behind their clients in the use of information and communication technology (ICT) such as social media, which could be a valuable tool for outreach and education. We surveyed sustainable agriculture stakeholders in California — extension professionals, county agricultural commissioners, and members of farm bureaus and producer groups — to measure their ICT behavior and attitudes. Drawing on diffusion of innovation theory, we characterized the innovation attributes of ICT that may influence the adoption and use of new technology among extension professionals. We also studied their demographic characteristics to establish whether there was a connection with ICT use. The main perceived benefit of ICT was that it can quickly reach larger, more diverse and more distant audiences. The perceived challenges included lack of professional support, the potential for misinformation on social media platforms, and the time requirements and technical complexity of technology use. Extension professionals experienced these challenges more than other sustainable agriculture stakeholders, creating a technology gap between extension professionals and their clientele. An ICT community of practice and clear organizational guidelines for measuring and reporting performance relating to ICT might help extension professionals close the gap.

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.