Advantages of using steam for soil disinfestation in strawberry production were presented at a February 2022 UCCE Strawberry Production meeting by Steve Fennimore, UC Cooperative Extension specialist in plant sciences.

Feasibility of using steam to kill soil pathogens and weed seeds in strawberry and other row crops has increased with new machinery development and field trials. As an alternative to fumigation, Fennimore said this sanitation treatment requires no buffer zones or plastic mulches used with chemical fumigation; however, current cost per acre for the propane and labor totals around $971.

An advantage of steam sanitation is that it lacks the negative environmental and worker health regulations associated with chemical fumigants.

Heat treatment for soil sterilization is not a new idea, but equipment availability and costs have been drawbacks to adoption. The machine in Fennimore’s presentation was built in South Korea.

The machine injects steam into the soil to raise the soil temperature to 158 degrees F for 20 minutes. The machine is moving while injecting steam, Fennimore said. The steam transfers heat from the heat source to target soil particles. When steam comes into contact with cold soil particles, the steam molecules condense, releasing heat to the soil particle. Steam kills the pathogen in and around the soil particle, and the steam also kills weed seeds and nutsedge tubers.

In a 2021 strawberry nursery trial, Fennimore said use of steam significantly reduced hand weeding costs for growers. Compared to untreated control ground, steam-treated soils yielded less than 272,000 weed plants while untreated control had 420,000 weed plants per acre. In strawberry nursery plant production, more daughter plants were produced under steam disinfestation.

In a production strawberry trial, steam treatment was as effective as PicChlor 60 in controlling soil pathogens and weeds. Fruit production from steam-treated soils equaled that of PicChlor 60 treated soils.

Band steam treatments in lettuce also proved effective for weed and pathogen control. In a Yuma, Ariz. trial, the steam machine operated at three quarters of a mile an hour and injected steam in a band where the lettuce crop was to be planted. The steam treatment reduced weed emergence and soil pathogens. It also increased lettuce size. A good fit for steam band technology, Fennimore said, may be organic production where there are few good disease and weed control options.

Fennimore reported increased interest in steam soil injection as a method of reducing weeds and soil pathogens. In addition to the South Korean machinery used in the trials, Fennimore said a Norwegian startup is also developing a soil steam injection machine and has expressed interest in bringing the machine to the U.S. for trials.

Orchard history, bud monitoring and the mathematical model Xanthocast are the three methods for attempting to predict or forecast walnut blight.

Sac Valley Orchard Source, a UCCE publication, notes that all three methods have advantages and disadvantages, and their use can be a guide to decision making when implementing a walnut blight preventative management program.

Walnut blight is caused by a bacterial pathogen that can overwinter in between scales of healthy buds. Rain can splash the bacterium onto developing flowers and leaves. Twig cankers are also an overwintering site that can supply inoculum for infection. Bud infections can result in bud death and fruit infections.

These infections can develop into twig cankers, which allow survival of the blight pathogen from one season to the next.

Orchard history involves surveying 10 trees for infected nuts the previous June. If fewer than 50 infected nuts are found, there is a low risk, and 50 to 150 infected nuts is a high risk. High disease levels can mean high bud populations and presence of twig cankers.

With bud monitoring, dormant buds are sampled for colony formation. Positive identification of bacterial species is needed. Orchard Source also notes that orchards can go quickly from low risk to high risk depending on weather conditions.

Xanthocast is a mathematical model that uses leaf wetness and temperature for forecasting emergence of the disease and timing of treatment intervals. Values should be combined with information from orchard history and bud monitoring.

The bactericide Kasugamycin, labeled as Kasumin, is used for managing walnut blight. According to Orchard Source, applications should be initiated when conditions favor disease development.

In orchards with a history of the disease and when high rainfall is forecast, applications should be initiated at 20% to 40% catkin expansion. Under moderate/low disease pressure, including low rainfall forecasts and minimal dews, applications should start at 20% prayer stage (leaflets unfolding, before expansion) and at 40% prayer stage when disease pressure is very low. These stages correspond to pistillate flower emergence.

In orchards with very low walnut blight infection rates or young orchards with no blight history, growers should consider a spray application at 40% prayer stage.

The newly launched Better Soils Alliance is offering an opportunity for almond growers and orchard managers to improve their efficiencies in water and nutrient use while demonstrating to buyers of almonds that they are intent on ensuring sustainability of the industry.

Announced at the 2021 Almond Conference, the Alliance uses the continual improvement concept and aims to verify improvement in resource use and reduction of greenhouse gas emissions by almond growers who participate.

Agricultural nutrient company Yara and food and agriculture technology company Heliae Agriculture initiated this alliance to determine management practices that will improve efficiencies in almond production and bring in growers who want to improve health of their soil.

Soil scientist Dr. Rob Mikkelsen, agronomic director for Yara North America, said the alliance is open to growers and others involved in the almond industry to incentivize improvements in water productivity and nitrogen use efficiency. The alliance will also be calculating greenhouse gas reductions that may provide alternative revenue streams for growers in the future. Validated and verified improvements would create opportunities for payments for carbon credits.

The Better Soil Alliance aims to provide greater support for growers and advisors by engaging all stakeholders to drive practical and profitable solutions in almond production.

Mikkelsen said precise and balanced use of nutrients in agriculture production could prevent soil degradation which happens over time as nutrients are mined from the soil.

Devin Clarke, permanent crop manager for Yara North America, indicated the need for a multi-tiered evaluation structure to reach more acres and conditions. This structure should dramatically decrease the time required to adapt to industry-wide challenges like water availability and consumer demands.

The Better Soil Alliance phase one efforts are focused on leveraging crop nutrition and soil biology to improve input use efficiency. The Better Soil Alliance will be conducting trials in the 2022 crop year to assess both improvements in plant-soil interactions but also yield, quality and grower profitability.

“Improving soil quality serves as the foundation to improving water use that helps optimize crop yield and quality”, said Norm Davy, Heliae Agriculture. “Between the increased consumer demand for almonds and tightening water supply in California, growers need solutions that increase yield and improve soil health and soil structure for future crops.”

UCCE Viticulture Specialist Kaan Kurtural’s study of cover crops under no-till systems in vineyards was done to determine if grape vineyard water use could be improved.

In a presentation for the virtual San Joaquin Valley Grape Symposium, Kurtural said the 2020 study was focused on finding the best management for vineyard floors using cover crop and tillage systems to preserve plant-available water in soils prior to the initiation of irrigation. He also evaluated cover cropping and no-till practices for their sustainability in regard to mitigation of greenhouse gas emissions and their combined effect on grapevine physiology.

Kurtural added that the study would provide growers recommendations regarding the use of novel low-stature permanent grasses in vineyard systems.

The study was done in Fresno with three cover crops, including a perennial grass (Poa bulbosa hybrid), annual grass, barley and resident vegetation under till versus no-till. The randomized study was done in a Ruby Cabernet on Freedom rootstock vineyard under drip irrigation.

Results noted were a 30% increase in mid-day leaf water potential in early spring and a 10% increase the following year with the perennial grass. When extrapolated to a hectare scale, the perennial grass cover crop demonstrated a decrease in soil respiration, one of the primary sources of carbon loss in agricultural soils.

Kurtural said the net carbon assimilation of the system grown with perennial grass under no-till management was enhanced compared to the system with annual grass or resident vegetation.

At harvest, there were no changes in berry mass, acidity or sugar content.

Kurtural also compared the Fresno study with a Napa County study.

In Napa, with a transition from tillage with resident vegetation or annual grasses as winter ground cover to perennial grass, he found no-till saves between 3% to 6% of annual cultural costs ($294 and $552 per acre, respectively.)

In Fresno, transitioning from tillage with resident vegetation or annual grasses as winter groundcover to perennial grass and no-till saves between 9% to 16% of annual cultural costs ($174 and $339 per acre, respectively.)

Kurtural said that cover crops under no-till systems may be implemented in the San Joaquin Valley irrigated vineyards with no effect on grape productivity and improvement in grapevine water use.

The remaining component of the research will include GHG flux calculation and characterization of bacterial and fungal communities.

Although Salinas Valley lettuce growers enjoyed a reprieve from virus pressure in 2021, the plentiful December rains have sparked intense weed growth, providing prime habitat for Western flower thrips, a vector for impatiens necrotic spot virus. The UCCE publication Salinas Valley Agriculture reports that this plant virus caused $100 million in lost gross revenue for Salinas Valley lettuce growers in 2020.

“It’s a blessing, yes, we need the water,” said Tony Alameda, managing partner of Topflavor Farms, which grows a variety of produce in Monterey and San Benito counties. “But, oh gosh, with that water, here come the weeds, here comes the habitat, here comes all the other problems that go along with it.”

The agricultural community called it “the biggest problem we’ve seen in a long, long time,” said Mary Zischke, facilitator of a task force convened by the Grower-Shipper Association to address INSV and a related affliction, Pythium wilt.

Even though 2021 was a “good” year, about one-third of all lettuce plantings in the Salinas Valley had at least a low level of infection, according to Zischke.

INSV on lettuce causes characteristic patterns of chlorosis and necrosis on the inner leaves of the plant as well as significant stunting. However, INSV can cause significant necrosis and lesions on and at the base of the ribs of lettuce plants. Lettuce plants infected with only INSV do not exhibit wilting of the outer leaves of the plant or show root rot or root discoloration.

Salinas Valley Agriculture reports that recent studies have identified several weeds as key reservoirs of thrips, including malva, marestail and hairy fleabane. Mustards, fortunately, appear to be poor hosts for thrips, although their pollen serves as potential food sources.

Aggressive weed management was an important factor in limiting the virus’ spread in 2021. Because weeds recognize no boundaries, managers of non-agricultural lands are being urged to keep their properties as clean as possible, including industrial sites, equipment yards and the edges of roadways, namely U.S. Route 101, which runs through the center of the valley. Some growers have been volunteering to weed their neighbors’ vineyards.

“We’re encouraging everybody, as best they can, to knock down known weed hosts; that’s really critical,” Zischke said.

Heat waves were a major driver of the INSV disaster of 2020. Researchers have established a link between warmer temperatures and population increases of thrips.

Alameda and Zischke point to the breeding of more resistant lettuce varieties as the ultimate solution to INSV.

With funding from CDFA’s Specialty Crop Block Grant Program, UCCE Irrigation and Water Management Advisor Ali Montazar will work on development of irrigation tools and strategies for avocado growers in Southern California.

Montazar, who works with growers in Imperial, Riverside and San Diego counties, said avocado growers in Southern California face uncertain water supplies and mandatory reductions in water use. Improved irrigation and water use efficiency are critical to sustainable avocado production. Efficient use of irrigation water is one of the highest conservation priorities, he added. Proper irrigation management is also crucial to managing tree nutrition as well as preventing Phytophthora cinnamomi or avocado root rot.

Avocado water demand may vary depending on canopy features, row orientation, weather, soil types and conditions, and irrigation practices. Different avocado varieties do not necessarily have different water demands while they might have different responses to water or salinity stress. The negative impact of stress can be different for different varieties.

Montazar said one of the main purposes of this study is to develop information on avocado water demands under different conditions. Deficit irrigation trials will be used to see responses in fruit quality and quantity.

New tools and resources developed in this project will help growers achieve water use efficiency goals, Montazar said.

The research team is collaboratively working with California Avocado Commission and several growers. In this project, a combination of field experiments, case studies and a robust outreach program are used to develop and disseminate information and tools to growers and stakeholders. These tools and information may have a significant impact on water quality and quantity issues, Montazar said, bolstering the economic sustainability of avocado production not only in the well-established production region of Southern California, but also in Kern and Tulare counties where new avocado plantings are growing.

With 26 fungal species associated with canker disease, it is important for growers and farm managers to determine which pathogen is attacking their almond trees. Field diagnosis plus a laboratory test can reveal the responsible pathogen or point to an abiotic reason for canker formation.

Florent Trouillas, UCCE plant pathology specialist at the Kearney Agriculture Research and Education Center, noted during a presentation at The Almond Conference that in order to use the most effective control options, it is critical to understand which diseases can affect the trunks and scaffolds of almond trees. UCCE Plant Pathologist Themis Michailides and USDA researcher Greg Browne also provided information on control and prevention of canker diseases.

Most infections of fungal canker diseases occur at pruning wounds made for primary and secondary scaffold selection. Fungal pathogens that cause gumming and dieback in almond trees are the leading cause of death in young orchards, Trouillas noted. These pathogens include Botryosphaeria, Ceratocystis, Diaporthe, Eutypa and Phytophthora.

Botryosphaeria is associated with band cankers that infect cracks in the bark and pruning wounds. These cankers often present in a row around the trunk. Two- to five-year-old trees with vigorous cultivars are most affected by this disease.

Phytophthora and Ceratocystis infections also cause gumming on the tree trunk and scaffold branches. Ceratocystis cankers are more elongated and are common in prune and cherry trees. The pathogen is transmitted by insects and affects both young and old trees. Phytophthora infections are notable for their quick development as well as gumming of the trunk and scaffolds.

Eutypa infections begin in pruning wounds or in cracks near the tree crotch.

There are also abiotic reasons for trees to develop cankers. Those include herbicide injury, acid burn and boron toxicity. Foamy canker, marked by copious amounts of reddish gum that flows from the cankers, can be caused by a number of abiotic stressors.

Location on the tree and infection sites can help with a field diagnosis, but Trouillas said a molecular diagnosis is a tool that can now be made in 24 hours using a species-specific primer that targets all canker pathogens.

Michailides noted that band canker in almonds has been on the rise since 2005 and has the potential to kill trees. Bands of cankers develop in a circular pattern around the tree trunk. In severe cases, two to three bands may be present.

Preventative measures in young orchards involve obtaining clean trees from nurseries. Once planted, the fungicide Topsin-M should be applied to the trunks at label rates during the first-, second- and third-leaf years. Avoiding wetting tree trunks during irrigation and protecting pruning wounds with Topsin at label rates is advised.

When band canker is present in young orchards, it is recommended to keep tree trunks dry, apply Topsin-M to trunks and scaffolds and to also protect pruning wounds. Killed trees and stumps should be removed from the orchard and wood piles should not be located near orchard sites to lower inoculum levels.

Best management of perennial Phytophthora Canker (PCC) and Phytophthora crown and root rots (PCRR) involves integrated cultural, genetic and chemical control.

Phytophthora, a ‘water mold,’ is adapted to being spread by surface water and reaching plant roots, persisting in adverse conditions.

Browne said that PCC, which mostly results from scion infections, is most prevalent in mature orchards, while PCRR results mostly from rootstock infections and is most prevalent in young orchards.

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New information on the invasive fruit fly spotted wing drosophila (SWD) comes from Dr. Artyom Kopp at UC Davis. Published in the January 2022 ANR Blog Strawberries and Caneberries, the information sheds light on origins of SWD, how and why it attacks fruit and possible biological control tactics.

SWD was found in 2008 damaging fruit in many California counties. It infests ripening cherries throughout the state and ripening raspberry, blackberry, blueberry and strawberry crops, especially in coastal areas. It also has been observed occasionally attacking other soft-flesh fruit such as plums, plumcots, nectarines and figs when conditions are right.

SWD, Drosophila suzukii, is native to East Asia and has been established in Hawaii since the 80s, where it is found in large numbers on guava and other fruit. In California, the first SWD were collected in 2008, and by 2009-10, they were a major problem in California, Oregon, Washington and Florida.

Unlike most fruit fly species, SWD has a prominent saw-like ovipositor that enables it to lay eggs in undamaged fruit. The larvae are tiny, white cylindrical maggots a little longer than 1/8 inch when fully grown. One to several larvae can be found feeding within a single fruit. After maturing, the larvae partially or completely exit the fruit to pupate.

All Drosophila species use a mix of sensory cues to identify places suitable for egg-laying. They smell with their antennae, taste with their legs and mouthparts, and measure hardness with their ovipositors. SWD prefer the smell of intact fruit and do not mind hard surfaces.

Kopp noted that once SWD damages fruit by opening the skin, the fruit is accessible to Drosophila species such as D. melanogaster, D. simulans, and D. hydei. These species are more abundant, more prolific, faster-developing and more heat-tolerant than D. suzukii, so they tend to out-breed it once it provides them with access to the fruit. Kopp said most flies hatching are probably those other species, but it was likely D. suzukii that let them in.

Several generalist wasps native to North America and Europe can parasitize D. suzukii under laboratory and field conditions. Some Leptopilina and Ganaspis wasps, abundant in Asia, seem to be specialist parasitoids of D. suzukii, preferring it to other Drosophila species. They prefer to attack the larvae that are feeding on ripening fruit and cause high mortality in D. suzukii in its native range.

Kopp said use of transgenic tools to introduce artificial constructs can suppress population growth. Transgenic tools for D. suzukii have recently been developed, and several female-killing approaches have been tested in lab trials. 

Extending the profitable lifespan of a table grape vineyard includes management practices to prevent trunk diseases, weed and nematode control.

The UC Integrated Pest Management guidelines for table grape vineyards note that delaying vineyard pruning until late in the dormant period is effective in reducing fungal infections. The delay avoids the highest spore release time of December and January.

Preventative practices are most valuable in young vineyards, but they have also shown some efficacy in older vineyards with prior trunk disease infections. The guidelines report that post-infection practices, including sanitation and vine surgery, are more expensive than preventative practices adopted early in a vineyard’s life. Those include delayed pruning, double pruning and use of pruning wound protectants.

Dormant-season sanitation practices include destroying prunings of older, infested wood to reduce pest sources. Dried grape clusters should be removed from vines and weeds disced to remove overwintering sites for orange tortrix or omnivorous leafroller. Where branch and twig borers have been a problem, old pruning scars and dead parts of vines should be scouted for evidence of brown frass and wood dust. Where vine mealybug has been found in a vineyard, care should be taken to prevent spread of this pest to uninfested areas of a vineyard by sanitizing machinery.

Weed control is essential in young vineyards until canopies are large enough to shade out competition. Surveys can allow for identification of weeds that escaped control in the last growing season and to determine which perennial weeds are present. Finally, surveys help with choosing appropriate herbicides of cultivation.

Cultivation may be preferred to herbicide use in young vineyards to prevent injury to vines. In mature vineyards, herbicide application in the vine row should be used together with mowing or cultivation between the rows. In the absence of herbicide applications, mowing may be needed when weeds exceed six to eight in height. Cultivation in the rows may be necessary after irrigation causes weed seeds to germinate.

In the San Joaquin Valley, sampling soil for nematodes is done from November to February when X. index or dagger nematode populations are most likely to be detected. Root knot nematodes are found at any time of the year. Soil samples should be sent to a diagnostic laboratory for identification.

Xiphinema index can cause yield reduction in some varieties but is more important for its transmission of grapevine fan leaf virus, the cause of grapevine fan leaf degeneration disease.