Wilt disease in strawberries caused by the pathogen Fusarium oxysporum forma speciales fragariae is expanding quickly and merits attention from strawberry growers.

Mark Bolda, UCCE strawberry and caneberry farm advisor in Santa Cruz County, emphasized that fumigation operations in infected fields are worth the cost even when using plants resistant to this pathogen. Speaking at the 2021 Crop Consultant Conference, Bolda also noted that crop termination works to improve yields in resistant varieties through the season and in the first half of the season in susceptible varieties.

“Crop termination very much looks like it enhances chloropicrin efficacy in susceptible varieties,” Bolda said.

The majority of the genus Fusarium oxysporum is not pathogenic and is common in soil and around roots. Individual types are specific to host plants. This pathogen does grow on other field crops, but does not thrive, except on strawberries. Infected plants become noticeable from May to June as crowns split and become discolored. Bolda said 80% of the disease load is in the crown of the plant.

Bolda said Fusarium in strawberry could possibly coincide with the reduction and prohibition of methyl bromide. When many plants are diseased at once in a field, that does not necessarily mean the disease came with the transplants. This specific pathogen does not show symptoms at low levels and can augment populations over several successive crops of strawberries. Management of this pathogen consists of sanitation, fumigation, crop rotation and possibly fungicide applications.

He advised minimizing movement of soil from field to field on shoes and equipment to keep from spreading the pathogen to uninfected fields. Crop rotation is at least 18 months before returning to strawberries. The longer the better, Bolda said.

Bolda’s field trial set out to test efficacy of fumigant treatments and crop rotations against the strawberry-specific Fusarium pathogen. Treatments tested were KPAM at 20 gallons per acre and crop termination, Dominus at 20 gallons per acres, Tri Clor 80 at 350 pounds per acre and KPAM drip at 47 gallons per acre. Varieties were Fusarium-resistant San Andreas, Fusarium-susceptible Monterey and Fusarium-resistant Fronteras. Dominus, Bolda noted, is close to being registered in California.

All of the treatments showed significantly higher crop yields in Monterey from April through August compared to the untreated control. The treatment of KPAM plus crop termination then Tri Clor 80 had the highest yields.

New insecticides, new combinations of insecticides, biological controls and improved mating disruption to control vine mealybug (VMB) are all under study, but costs and necessary control levels are issues, said UCCE Assistant Specialist Kent Daane.

The invasive vine mealybug is the most problematic of all mealybug species and causes the most damage in California vineyards. Damage by the vine mealybug is similar to that of other grape-infesting mealybugs in that it produces honeydew that drops onto the bunches and other vine parts and serves as a substrate for black sooty mold. Mealybugs also will infest grape clusters. Vine mealybug can vector grapevine leafroll associated viruses which have been associated with sudden vine death.

During dormancy and early spring, VMB are found primarily on the trunk, canes and roots, but this can vary by vineyard location. In the spring, VMB moves onto canes and later onto leaves. In the summer, VMB moves into grape clusters. The trunk and the bark of canes provide refuge from insecticides, including systemics and natural enemies.

Ants can be a sign of a VMB infestation. Daane said that ants give refuge to VMB and also improve their habitat by removing honeydew. This tending by ants, increases VMB populations.

Bark wetness is a sign of a mealybug infestation as honeydew production increases. Leaf drop in June can also be sign of an infestation. If a VMB infestation is suspected, crowns and trunks can be inspected in the spring for adult females and crawlers. Starting at bloom, cordons, canes and basal leaves should be inspected. When fruit is present, clusters can be scouted for signs of VMB. Pheromone traps in vineyards from August to October can give record of VMB numbers and indicate numbers for next season. Daane warned that it is common to have high trap counts with little actual crop damage.

Increasing resistance to chemistries used in the past to control VMB make it important to rotate those chemistries which are still effective. Daane said there are both conventional and OMRI-approved insecticides to control VMB, but none provide 100% control.

“Movento is the best, but we are seeing cracks in control,” he warned.

Use of mating disruption devices in the vineyard can help with control. They work best with low VMB pressure. Daane said the effect is better in the second or third year. Ants also must be controlled.

The California Department of Pesticide Regulation (CDPR) is in the process of developing a statewide pesticide application notification system. Roger Isom, president of Western Agricultural Processors Association, participated in a DPR focus group session in August and reported on this issue at the 2021 Crop Consultant Conference.

Input from the agriculture industry on this proposal to require notification of a pesticide application is vital, said Isom. He urged all growers and PCAs to voice their concerns about the draft proposal to CDPR. Public webinars on the proposal are being held this month. Isom said the draft regulation would be announced in spring 2022.

Unknown at this time are the questions of who will be notified of a planned pesticide application, when will they be notified, how do they get notified, what products are covered and what application methods are covered.

At this time, it is only a proposal, and details are not decided, but Isom warned that the governor has indicated that notification is a priority for him and $10 million has been directed to develop the proposal.

Isom said this action was triggered by Assembly Bill 617 and an air quality reading in Shafter in Kern County that measured 1,3-D; however, there was no known application within seven miles of the measurement. AB 617, signed by Governor Newsom in 2017, establishes a community-scale emissions abatement program and updates air quality standards for sources that contribute to poor air quality.

Isom pointed out that only three states have some type of notification system for pest control applications: Florida, Maine and Michigan. Monterey County has a program for fumigation application notifications for schools. The public is eligible for notification and it is done five days prior to a fumigation. Currently in Kern County, notification is only for restricted use materials and is only required to growers in surrounding areas of the planned application.

Isom noted that in some instances, anyone is able to sign up for notifications, and in the past, it was found that the majority of those requesting notification did not live in the area of the application. In Florida, those requesting notification must be on adjacent or contiguous property up to a half mile.

Isom said any form of this rule should be statewide, reasonable and justification for those asking to be notified of a pesticide application. Public webinars are being held this month. Written comments can be submitted to CDPR.

UC Davis Plant Pathologist Themis Michailides reported recent research findings on walnut mold and management at the 2021 Crop Consultant Conference.

The main pathogens of walnut mold are Alternaria, Fusarium species and Aspergillus niger. Botryosphaeria and/or Phomopsis canker and the bacterial disease walnut blight are also cause for mold in orchards.

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Brown Apical Necrosis is a special type of walnut mold that always starts from the stylar end of the nut and is sometimes associated with the bacterial disease walnut blight. Michailides confirmed that there is a strong correlation of fungi causing walnut mold with the same fungi attacking and decaying the hulls. The incidence of mold starting from the stylar end was significantly greater than that of mold starting from the stem end, suggesting possible stylar infection at bloom. In trials over two years, sprays at two to three weeks before hull split and early hull split reduced walnut mold in both early and late walnut cultivars.

In the U.S. standards for grades of walnuts, mold is listed as damage when attached to the kernel or when white or gray mold affects a portion of the surface of the kernel. Potential for mold development in a walnut crop increases as harvest is delayed. Sunburnt nuts, windfalls on the ground for more than 15 days, nuts infested with navel orangeworm, nuts with large stem openings and larger nuts are also subject to mold development.

Michailides said levels of mold reported in recent years included a Chandler sample from Tulare County showing 21% mold, grade sheets from 2020 from Butte County areas near the Sacramento River with 10% to 16% mold and, in 2019, reports of 10% to 20% mold in Butte and Glenn counties.

In his presentation, Michailides noted that previous studies on walnut mold showed that mid-May to mid-July sprays reduced Botryosphaeria and Phomopsis canker and blight, but they did not reduce molds caused by Alternaria or Fusarium. He said the fact that fungi-causing mold are the same as those that colonize the hulls led the decision to move towards sprays before hull split and early hull split.

To reduce incidence of mold in walnuts, Michailides said applying Merivon at three weeks prior to hull split reduces mold related to Botryosphaeria, Phomopsis and Alternaria. Adding Tebuconizole to the tank mix will increase efficacy against Phomopsis. To further increase efficacy, apply Rhyme at 20% to 30% hull split. If the high level of control is not needed, Rhyme can be applied at 20% to 30% hull split.