Diablo Region Crops Blog

Autumn is in the air, and while it is still high season for harvest for many crops, shorter days and cooler temperatures inevitably bring to mind that winter prep tasks are not far off. And while the winter is the off-season for most crops in our region, it is definitely the on-season for cover crops. This is because, in California where irrigation water is a premium resource, we typically rely on winter rains to water cover crops. While it is ideal to make plans in advance and get the seed order in ahead of the fall rush, there is still time to think about getting a cover crop in before winter storms settle in.

What are cover crops and what are their benefits?

Cover crops include a wide range of herbaceous plants grown with the primary goal of producing biomass for the soil. They offer many opportunities to address specific soil concerns, in addition to improving and maintaining soil health, in general.

Cover crops can:

• Improve soil physical & biological functions
• Improve water infiltration
• Contribute and retain soil nitrogen 
• Allow machinery earlier access to wet fields
• Suppress weeds
• Support pollinators & other beneficial insects

Water infiltration is one of the main issues that motivate growers to plant cover crops in California, as roots open new channels and feed underground life. This is especially helpful in orchards and vineyards where soil compaction is especially difficult to remedy. Cover crops can also contribute significantly to nutrient management, both in the use of nitrogen-fixing legume species, or by just foraging left-over nitrogen at the end of the season and keeping it from leaching out of the soil profile. The organic matter that cover crops contribute also improves soil structure and how much water and nutrients it can hold.

Cover crop types and their applications

Cover crops are often broadly grouped by plant types that differ in their primary benefits. For example, grasses generally produce dense stands with extensive, fine, fibrous roots that improve water infiltration and are excellent at mining nitrogen. They are more amenable to driving on at earlier growth stages, and are usually easy to mow. Legumes can actually add nitrogen to the soil through their symbiosis with bacteria that sequester nitrogen from the atmosphere. Because of their high nitrogen content, their biomass generally decomposes more quickly than other cover crops, which can mean a quicker turnaround in a tight planting schedule. Plants in the mustard family produce deep, thick tap roots that open up the soil, and their flowers are favorites of pollinators and other beneficial insects, but if allowed to mature, their sturdy stems may take more work to chop and more time to decompose.

These different types can be used singly or in combinations. There are mixes that incorporate all three types to "hedge bets" against conditions that might disfavor one or another type, and to take advantage of the way that they can complement one another. Many seed companies have developed their own mixes with characteristics to serve specific needs and circumstances.

2022 Orchard Cover Crop Trial

Cover cropping can be as much art as science, if not more so, which means that  experience and experiment  are essential. To this end, in the fall of 2022, I teamed up with Tom Johnson of Kamprath Seed and a local walnut grower to test out some different cover crops with the hope to improve water infiltration in the orchard. Our trial included a winter triticale, a hybrid grass known for abundant root production without too much aboveground growth, a mustard “pollinator blend” that included a mix of mustard and radish types, and Kamprath Seed's walnut mix, which includes a full complement of grasses, mustards, and legumes. The grower added a forage, spring triticale, which has more of an upright growth, adding a second triticale type for comparison. It turned out to be a lucky year to have cover crops in the orchard, as it was an exceptionally wet winter, putting many fields feet underwater, especially where infiltration was poor. Overall, all of the cover crop types established rather well in most of the planted rows, though there was a great deal of variation throughout the orchard, showing how conditions can affect the crop. Nevertheless, the walnut mix (with legumes, mustard, and grasses) consistently produced the highest total biomass, even in places where the mustard mix and straight triticale came up somewhat thin (see chart below). This highlights the advantages of diversity, but as Tom pointed out during our spring field day for the trial, results may vary considerably from year to year, and each type or blend will have advantages and disadvantages. You can learn more about the trial and the qualities of different cover crops in a video version of the field day on my YouTube channel.

Getting started with cover crops

If you are inspired, I think it is with good reason. Cover crops are tools with great potential and they really engage farming know-how and problem-solving muscles. As in all new things, it's always best to start small and experiment. There are many resources available for learning the basics or refining your existing knowledge about how to effectively employ and benefit from cover crops. In 2021, UCCE and the Resource Conservation District in Contra Costa County together developed Cover Cropping Opportunities in Specialty Crops, a series of recorded webinars and virtual site visits that explore the world of cover cropping from many different angles. These videos offer a wealth of information, instruction, and demonstrations from experienced practitioners and experts.

In addition, below are listed some other excellent UC resources on cover cropping:

Cover Cropping Opportunities in Specialty Crops video series by Contra Costa County Resource Conservation District and UC Cooperative Extension

Grasses, Mustards, & Legumes: An Orchard Cover Crop Field Day video featuring Tom Johnson of Kamprath Seed by UCCE Specialty Crops in the East Bay/Mt Diablo Region

Covercrops for California Agriculture UC ANR Publication Number 21471

Cover Cropping for Vegetable Production UC ANR Publication Number 3517

Cover Cropping in Vineyards: A Grower's Handbook UC ANR Publication Number 3338

Cover Crop Management in Annual Farming Systems Blog Post by UCCE Agronomy Advisor, Sarah Light

Cover Crop Best Management Practices published by the Almond Board of California in collaboration with UC Davis and UC ANR.

Cover Crops for Walnut Orchards UC ANR Publication Number 21627e

Cover Crop Selection tool by UC ANR SAREP

(Este post está disponible en español)

In early September 2023, nearly the entire growing area around the city of Brentwood  was put under a quarantine by the California Department of Food and Agriculture (CDFA) (see current included area). This severe regulatory measure was triggered by numerous finds of the oriental fruit fly by Contra Costa County's Department of Agriculture during routine monitoring. This exotic pest (meaning that it is not established in the U.S.) is on the United States Department of Agriculture's (USDA) list of non-native species that pose the greatest potential threat to agriculture and the environment. Given the strict and potentially costly treatment and record-keeping requirements in the middle of a busy harvest season, and, for some in the growing community, outright losses, many are likely wondering why this pest is so great a concern.

What is the Oriental fruit fly and why is it a threat to agriculture?

The oriental fruit fly (OFF,  for short),  known  scientifically as Bractrocera dorsalis, is in the same family (Tephritidae) as the olive fruit fly, but has a much greater potential to harm the agricultural economy of the county and of the state. The difference is largely based on the number of crops that OFF can damage. While the olive fruit fly essentially affects only olives, OFF is able to attack more than 230 crop species.

These include most fruit crops grown in the Brentwood area and in California, in general, including most pome and stone fruit (apple, apricot, cherry, peach and nectarine, pear and plum), most citrus fruit, grape and walnut, as well as specialty market fruits such as avocado, fig, persimmon and pomegranate, to name a few.

Vegetable fruits are also susceptible, including bell pepper, cucumbers, melons and squashes and tomatoes. (Curiously, almonds, olives, strawberries and pumpkins* are not considered hosts, and are not under quarantine restrictions.) Tropical fruits, like guava, mango, and papaya, which are not grown in our region but often sold in open markets, are also favorites of the pest. (See this link for CDFA's abbreviated host list.)

OFF is considered to be one of the most destructive pests in its current range, which includes Southeast Asia, where it appears to have originated, and sub-Saharan Africa, where it has spread rapidly after being introduced only two decades ago. In Hawaii, where it was introduced in the 1940s and subsequently became established, it is a major pest of nearly all commercially grown produce. The combination of a high potential to damage fruit and a very wide host range, as well as its great reproductive capacity, make this pest one of the highest priorities for exclusion from California and other parts of the mainland United States. In addition to costs associated with direct losses of damaged fruit and new management treatments, if this pest were to become established, it would severely impact export markets for nearly all major commodities. Due to its destructive potential, the pest is under aggressive management where it occurs and is subject to exclusion measures in many regions including Europe, Africa and Asia. 

Like other fruit fly pests, the damage done to fruit by OFF is caused by the larval stage. Female flies insert eggs into the flesh of ripe or, less preferentially, green fruits using a piercing organ similar to a stinger called an ovipositor (see photo below). The worm-like larvae feed on the fruit pulp under the skin until they are ready to pupate, at which point they eat their way out of the fruit and drop into the soil. In addition to the risk that consumers find maggots (the larval stage) in the fruit, the damage they cause in fruit make it more susceptible to secondary decay by fungi and bacteria. OFF can go from egg to adult in a little over two weeks, and can be ready to reproduce again in about 30 days, and a single female may lay more than 1,000 eggs in its lifetime!

Regulatory efforts to exclude high impact pests

OFF is one of several exotic fruit flies regulated by the federal government and that are monitored by county agricultural departments in coordination with the state, as part of their mission to protect the state's agricultural producers and the environment from new pests and diseases. Other exotic flies actively under exclusion efforts are Mediterranean fruit fly (MedFly) and Mexican fruit fly. A similar quarantine and eradication were implemented in Solano County for MedFly a few years ago. Unlawfully imported fruit are considered the main way that these pests find their way into the country.

OFF is regularly detected in California, but, fortunately, many quarantine and eradication efforts in the past have been implemented successfully. Most quarantines have been in urban areas that did not substantially affect agriculture in surrounding areas. Unfortunately, the finds in Brentwood have affected significant acreage in and around the city. This is perhaps another challenge of the urban-agriculture interface in our region which otherwise offers many benefits, like the thriving U-pick business. A quarantine and eradication effort have also been put in place for Santa Clara County since late August. Eradication treatments entail the attraction of male flies to "bait stations" containing an attractant and a biological insecticide, which is approved for use in organic agriculture. The process is described in some detail in the CDFA's OFF fact sheet.

What this quarantine means for Brentwood area growers

Growers in the quarantined area (see the map linked here for current boundaries) cannot sell or ship produce off of their property without a compliance agreement. The Contra Costa County Department of Agriculture held a joint meeting with the CDFA at the Knightsen Farm Center on September 7 to explain the quarantine and compliance requirements.

Compliance requires growers to make a certain number of treatment applications on a specified schedule under the supervision of state or county agents The Department of Agriculture will provide details on compliance requirements and treatment options. During warm, summer conditions, treatments can be completed in as little as 30 days, but as we go into the cooler, autumn months, the required treatment window expands due to slower development of OFF. Therefore, it is critical to begin the compliance process with the agriculture department as soon as possible. For produce destined for processing, such as processing tomatoes, winegrapes and walnuts, the requirements are somewhat more flexible, but it is still essential to contact the agriculture department to set up a compliance agreement as soon as possible before harvest.

There are treatment options for organic growers. The quarantine is expected to extend at least through the beginning of next year's growing season, so it is essential for growers to plan ahead for next year's crop, as well. 

More information is available at the resources listed below.

* Note that though pumpkin and related types of Cucurbita pepo appear on the comprehensive host list, CDFA authorities have stated that this species is no longer regulated. Please check with the Contra Costa County Department of Agriculture.

 References and Sources:

• Oriental Fruit Flies Detected in Contra Costa County Quarantine Forthcoming Contra Costa County Department of Agriculture News Release August 30, 2023
• Oriental Fruit Fly Regulation and Quarantine Boundaries, California Department of Food and Agriculture
• Oriental Fruit Fly Fact sheet, California Department of Food and Agriculture
• Oriental Fruit Fly Pest Profile, California Department of Food and Agriculture
• Current Exotic Fruit Fly Quarantines in California, California Department of Food and Agriculture
• Featured Creatures University of Florida IFAS
• State Takes Emergency Action to Eradicate Oriental Fruit Flies Found in City of Santa Clara. County of Santa Clara Office of Communications and Public Affairs

Scientific articles:

• Capinera, John L. “Oriental Fruit Fly, Bactrocera Dorsalis (Hendel) (Diptera: Tephritidae).” In Encyclopedia of Entomology, edited by John L. Capinera, 2690–92. Dordrecht: Springer Netherlands, 2008.https://doi.org/10.1007/978-1-4020-6359-6_1882.
• Mutamiswa, Reyard, Casper Nyamukondiwa, Gerald Chikowore, and Frank Chidawanyika. “Overview of Oriental Fruit Fly, Bactrocera Dorsalis (Hendel) (Diptera: Tephritidae) in Africa: From Invasion, Bio-Ecology to Sustainable Management.” Crop Protection 141 (March 2021): 105492.https://doi.org/10.1016/j.cropro.2020.105492.
• Wei, Dong, Wei Dou, Mingxing Jiang, and Jinjun Wang. “Oriental Fruit Fly Bactrocera Dorsalis (Hendel).” In Biological Invasions and Its Management in China, edited by Fanghao Wan, Mingxing Jiang, and Aibin Zhan, 267–83. Dordrecht: Springer Netherlands, 2017.https://doi.org/10.1007/978-94-024-0948-2_15.