October 2015 Research

Research on the Central Coast of California By Steve Tjosvold

The past 15 years have been dominated in the horticultural headlines by two invasive pest problems in the central coast of California and other areas — Phytophthora ramorum, the causal agent of Sudden Oak Death (SOD) and Light Brown Apple Moth, a leafroller with a very wide host range. As they began to impact the nursery industry, our research team has developed and promoted research-backed information on the biology and management of these new pests.

Sudden Oak Death: Biology and Management in Nurseries

In the mid-1990s, SOD was detected in Marin County (north of San Francisco), then by 1999 in the Santa Cruz Mountains (Santa Cruz County) and Big Sur (Monterey County). Thousands of oaks and tanoaks were killed in these areas. The disease spread to oak woodlands in other parts of northern California and southern Oregon, where it continues to spread today.

In 2000, 2002 and more widely in 2004, P. ramorum was detected in California nursery stock, and the industry was implicated in the unintentional movement of the pathogen to other states and Canada. A Federal Regulatory Order was established that required certification that all nursery stock in California be free of the pathogen. Nursery growers and regulatory officials needed to quickly understand the biology and epidemiology of the disease as it related to nursery stock to make prudent management decisions.

SOD research. Our research team first worked on the biology and management of the disease; first for woodland/urban interface issues, and then for nursery issues as they gained a higher priority. We found that P. ramorum could be detected in our local streams with a simple pear-baiting technique using a 3-gallon sample of stream water.

Currently similar baiting techniques are being employed by regulatory officials and scientists to monitor woodland streams, waterways and nursery water catchments throughout the United States and Europe. We found that hiking shoes frequently became contaminated with P. ramorum when hikers walked infested trails during the rainy season. This demonstrated that hikers could potentially move the pathogen long distances with contaminated hiking shoes. Educational materials at state parks and other natural areas, where SOD exists, prescribe the sanitation of hiking shoes before leaving the site. These findings also alerted nursery operators of the potential movement of the pathogen in a nursery with contaminated work boots.

Phytophthora ramorum becomes important for the nursery trade. We discovered the disease in a commercial rhododendron nursery in Santa Cruz County — the first in North America — in December 2000. This finding demonstrated the threat of nursery stock moving the pathogen and potentially infesting new areas. We evaluated the susceptibility of rhododendrons and azaleas. Of those tested, most evergreen and deciduous rhododendrons were very susceptible, and azaleas were susceptible but symptoms were weakly expressed.

Phytophthora ramorum in water and soil. We increased the understanding of how the pathogen moves and infects plants in a nursery during rain events by artificially infecting plants and infesting leaf litter in a simulated nursery and then following disease occurrence. Intra-plant aerial movement and infection was common, but movement between plants were usually in short distances, occurring in jumps only up to 0.5 meters in blocked rhododendron nursery stock. Inoculum from artificially infested soil only caused infections on lower leaves or leaves touching infested potting soil.

These and other experiments demonstrated that P. ramorum was moved more likely by splashing rain drops. Wind may have assisted the movement of the pathogen from the source of inoculum by driving infested rain drops, but this did not lead to actual plant infection at long distances from the source of inoculum.

Understanding how important infested leaf litter could be in the long term survival of P. ramorum proved important. P. ramorum was recovered from infested leaf-disks placed just under the soil surface in planted pots for more than one year; and these leaf-disks were still capable of producing infectious spores in flooded conditions. We saw that this pathogen has a potent weapon for survival! In the same experiment, root infections were detected 40 weeks after the infected leaf-disks were placed in the pots. Root infections are often asymptomatic and other researchers have found infected roots can produce infectious spores. We believe this may be an important — and difficult to detect — source of inoculum being moved in the nursery trade.

Phytophthora ramorum in irrigation water can infect rhododendron. A four-year study showed that local stream water, when used for sprinkler irrigation in a simulated rhododendron nursery, contained enough naturally occurring inoculum to cause disease. But the incidence was low and the risk only occurred in the spring when streams had relatively high levels of the pathogen and the cool, wet weather supported infection and development of disease. This raises concern for using stream water for irrigation in other areas in northern California, Oregon and Washington where streams can be perpetually contaminated with P. ramorum.

Phytophthora ramorum inoculum found in nursery runoff in all seasons. In another experiment, rhododendron and camellia were inoculated at four seasonal inoculation periods. We found that the highest concentrations of inoculum were found in the fall (October to December) and winter (January to March). Inoculum could be found in all seasons in runoffwater from sprinkler-irrigated plants up to the last evaluation (three months after inoculation) and in leaf washes up to the last evaluation (six months after inoculation) for each of four seasonal inoculation periods. High concentrations of inoculum in runoff-water and leaf washes were more commonly found after rainfall events than after sprinkler-irrigation events. In a follow-up experiment we found that sporulation increased logarithmically as leaf wetness hours increased. Moreover, this result remained the case through all field temperatures, except sporulation fell off the cliff when maximum daily temperatures were about 91o F and hotter. Production and movement of pathogen propagules in runoff appear to be very likely throughout the year. This poses an especially high risk if runoff is collected and recycled on plants without proper sanitation treatment.

Fungicides. We evaluated and found useful fungicides to help manage the disease in nursery crops. It turns out that most of the conventionally used fungicides used by the ornamental industry are also active on preventing foliar P. ramorum infection, and some such as mefenoxam (Subdue Maxx) and cyazofamid (Segway) were also active in preventing sporulation.

Light Brown Apple Moth Management in Ornamental Crops

In February 2007, the light brown apple moth (LBAM) — an invasive leafroller from Australia with one of the broadest host ranges known for any insect — was first detected in the San Francisco Bay Area. Federal and state quarantines are currently set in place to ensure that nurseries are free of LBAM before nursery products can be shipped to unregulated areas. If LBAM is detected in nurseries by regulatory officials, then growers usually must apply insecticides until LBAM life stages are no longer detected.

So far, growers in Santa Cruz and Monterey counties and 19 other counties in California have been affected by this quarantine. Unfortunately moths remain uncontrolled in urban landscapes and natural areas just outside of many nurseries, and in some situations, moths can migrate into and re-infest nurseries. LBAM eradication in these situations is very difficult. Since 2009, our research team has supported the development and use of management tools with a goal of driving LBAM numbers as low as possible in nurseries.

Effectiveness of LBAM pheromone mating disruption and traps. Disruption and reduction of LBAM mating by mass application of pheromone dispensers has been successfully employed in Australia and New Zealand in vineyards, citrus orchards, and in other large-scale applications, generally greater than 10 acres. What had not been demonstrated was whether the same management tool could be used in commercial ornamental nurseries that have many diverse crops and are less than 10 acres, as is the case in the Monterey Bay Area and many other areas in California. Our research team evaluated pheromone mating disruption in commercial ornamental nurseries and developed effective bait traps that could be used for monitoring purposes. We found that mating disruption was effective in reducing LBAM in nurseries but did not eradicate it. We found that the commercially-available pheromone dispensers would last at least six months in summer or winter conditions. We believe that mating disruption can be an economical component of an integrated pest management program but should not be considered a silver bullet.

Insecticides. The California Department of Food and Agriculture (CDFA) established approved insecticides that could be used in nurseries under quarantine resulting from official detections. These insecticides were based on laboratory and greenhouse tests, but there were no studies to evaluate their efficacy in field conditions. Moreover, growers needed to know how long these insecticides remained biologically active in the field so they could judge how often spray treatments needed to be applied.

There were two major studies, each consisting of multiple-year, repeated field tests. Most of the 13 tested products or combinations were effective in controlling LBAM. Residual effectiveness was determined to be less than one week to more than three weeks when applied before or after eggs were laid. Of the registered products in California, methoxyfenozide (Intrepid) was one of the most effective with residual activity of around three weeks. In addition, this product is selective to moth larvae and therefore does not disrupt beneficial insects when applied in a nursery.

Since older larvae are more robust and usually sheltered in leaf rolls, we wanted to know whether plant-systemic insecticides could penetrate leaves and kill larvae where they hide. These experiments evaluated several systemic and other insecticides on three age classes of larvae (young to old). In general, acephate (Orthene), emamectin benzoate (Enfold), methoxyfenozide (Intrepid) and cyantraniliprole (Mainspring) were the most effective on all larval stages. Dinotefuran (Safari) and spirotetramat (Kontos) provided some, but inconsistent, control on all larval stages. Grandevo, a new bacterial toxin that is organically registered, provided moderate control of the youngest larvae, although this was tested in only one of the two years. (Some of these insecticides were not registered in California.)

Improve scouting (field inspection) and identification of LBAM. In collaboration with LBAM experts at CDFA and Colorado State University, a practical field identification guide and training video was developed. Field inspectors and nursery scouts now have an aid to identify LBAM larvae and look-alikes in the field. See www.ipm.ucdavis.edu/EXOTIC/index.html.

Monitor and identify host species on ornamentals, natives and weeds on nursery perimeters. To help field scouts in pinpointing high risk hosts and to understand LBAM population dynamics, we monitored the perimeters of eight local nurseries and berry fields in Santa Cruz and north Monterey counties. Our data showed moth emergence relatively synchronized in an early November peak in all monitored areas. Moths emerged and larvae were present on various hosts during the entire year. There were 31 different weed and native host species that were identified. The data for moth emergence and a complete host list was updated every two weeks on the UCCE Santa Cruz website until the conclusion of the project in July 2014. Growers control higher risk weed hosts on field perimeters and intensify scouting when trap data indicate that moths could be migrating into their fields. This led to better control of LBAM in production fields. For more information see: cesantacruz.ucanr.edu/Invasive_Pests/.