One of the first posts to this blog regarded the importance of chill, both field and supplemental, to the day neutral (ie Albion, San Andreas, Portola, Monterey among others) strawberry varieties.  Right now, field chill in MacDoel is in the area of 600 hours, which is plenty, even in the light of the very warm stretch of weather that took place in September.  Still, be reminded that this abundance of field chill should not be considered to be a replacement for supplemental chill taking place in the cooler after the transplant has been harvested.

However, this year because of the delay in transplant harvest growers really should be striving to strike a balance between getting adequate supplemental chill to obtain good vigor and planting sufficiently early to get sufficient plant growth here in the fall.  To accommodate this idea, every one of the day neutral varieties listed above can be well established with supplemental chill of 7 to 10 days, but no less.  More days of chill, up to 18 days, are of course in the printed recommendations, but this year a large delay in planting may not result in acceptable plant growth and establishment.

If one follows the suggestion given above, supplemental chill will end up being on the low side, and subsequently growers must be more vigilant than usual in planting practices.  Transplants must not be allowed to dry out in the field during planting, transplants should be properly placed the planting hole (no “J” rooting, and only portion of the crown above the soil line) and irrigation for establishment should keep the beds at field capacity for a few weeks.

Introduction:  As the planting season for strawberry approaches, it is useful to review some concepts of strawberry plants and planting. Strawberry growers occasionally have questions about the size of strawberry transplants (i.e. crown size) and its effect on plant growth and yield. 

The following is a summary of a study run over two years as an investigation of whether strawberry crown size had any effect on strawberry fruit yield and size.

For those who want to see the full report, it starts on page 5 of the Monterey Crop Notes for November/ December 2007.

 http://ucanr.org/cropnotestransplant2007

Materials and Methods: 

Treatments:  The trials were conducted in a well managed organic farm in Moss Landing, CA.  In year one, the study consisted of treatments of “large” and “small” size transplants.  Measurements were done with a caliper at the middle of the crown. The following varieties were tested at the sizes given below:

Seascape: small < 1.1 cm, large > 1.2 cm
Transplants were dug 10/27/04 in Macdoel, and planted 11/16/04.

Aromas:  small < 1.1 cm, large > 1.3 cm
Transplants were dug 10/26/04 in Macdoel, and planted 11/9/04.

In year two, the study included a third treatment, consisting of “slicks” (known as “lisas” in Spanish) for the varieties of Seascape and Albion.  “Slicks” are very small crowns which are almost entirely made up of primary roots and no secondary roots, giving them a smooth appearance (see photo accompanying this article).

Seascape:   “slicks”, < 1.1 cm, large > 1.2 cm
Albion:  “slicks”, small < 0.9 cm, large > 1.1 cm
Both Seascape and Albion transplants were harvested in MacDoel 10/25/05, and planted 11/22/05

It should be noted that the large crowns in each treatment were not a common feature.  In each box of 1000 crowns large crowns represented probably 5-10% of the total numbers of plants.

Plants were measured for plant diameter early in the season, possibly on a monthly basis.  Fruit yield, both count and weight of fruit from each replicate plot, were evaluated by harvest by qualified personnel harvesting on a regular schedule. 

Results:  In the first year trial, plants of treatments planted to small crown sizes had smaller diameters than larger crowns one and a half months after transplant.  Three months after transplant this difference had disappeared.  There were no significant differences in plant diameters at any date in any size categories for the varieties tested in year 2.

Fruit production was significantly lower in smaller crowns of Aromas in the summer months of year 1, and resulted in lower total fruit production.  There were no differences in fruit production in year 1 or year 2 for Seascape.

For Albion, fruit production for "slicks" and small crowns was significantly lower than large crowns in the months of June and July, but there were no differences in total yield between any size class.

With very little exception did fruit size vary significantly between any of the crown size treatments by month or through the season.

Discussion:  Although there was some variation by month for certain varieties, transplant crown size was not a good predictor of total strawberry fruit productivity in this study.   What this means is that, while there was some cyclicality in production for some varieties, size of the strawberry transplant did not have an effect on total fruit yield.

                                        Mark Bolda
                                      Hillary Thomas


The use of pheromone based twist ties has found significant success in reducing the incidence of light brown apple moth mating in caneberries grown on the Central Coast of California.  With the recent advent of light brown apple moth infestation in area strawberries, especially those in organic production, there has been a groundswell of interest on testing the same technology there.

The following is a brief description of a trial mounted on the part of the authors in testing whether the use of pheromone based twist ties could be useful and successfully applied in strawberries.  On June 15, twist ties wound around chopsticks were distributed evenly (every other strawberry row with 16 to 18 feet between each stick) across 8 acres of organic strawberry at the rate of 300 per acre (higher end recommended label rate), with another 8 acres left alone as an untreated check.  This was replicated 4 times in various fields around Watsonville.

A matrix of twelve traps using pheromone lures was placed in each 8-acre field to test the ability of the pheromone based twist ties to prevent males from finding females emitting the same pheromone.  The idea is that if the males are unable to locate the pheromone lure in the trap, they will not be able to locate females and subsequently not be able to mate, resulting in population reduction.

Traps were checked yesterday, June 29.  Out of the 48 total traps placed over the four treated plots, only a single male moth was captured, while the traps in the untreated check contained two moth males on average.

Trap Captures of Light Brown Apple Moth Males, June 29, 2010
Totals are from 12 traps in each plot 

         with pheromone        without pheromone

Field 1              1                      23                   

Field 2              0                      27                   

Field 3              0                      23                   

Field 4              0                      42                   

The results suggest the twist ties may be useful as part of an integrated strategy to reduce light brown apple moth populations in fields, especially for organic growers who have limited available management tools.

by Steven Koike
UC Cooperative Extension

Starting around mid-May and extending into June, strawberry growers and pest control advisors in coastal California are observing the fruit problem known as bronzing. This problem occurs every year to some extent but can result in large economic losses in some seasons. Bronzing results in a tan or bronzed discoloration on green and ripening strawberry fruit. Bronzed fruit have dried, rough surfaces that render the fruit unmarketable (Photos 1 and 2). The skin of such fruit can later crack.

There are three types of bronzing. Type I bronzing occurs on distinct, localized parts of the fruit, often beneath the fruit calyx or around the strawberry seeds (achenes), and is caused by insect feeding, primarily thrips (physical damage due to abrasion (Photo 3) is also very localized). Type II bronzing is caused by chemical sprays that cause a one-sided bronzing to the side of the fruit exposed to the application. In contrast, Type III bronzing covers virtually the entire surface of the fruit, occurs during certain periods of time, and can result in devastating crop loss. It is notable that Type III bronzing in coastal California tends to occur when the weather is sunny and warm.

Field-based research has demonstrated that Type III bronzing is associated with fruit exposure to stressful environmental conditions that include extreme solar radiation, high temperatures, and low relative humidity. This Type III problem is not caused by sulfur applications or feeding by thrips, mites, or other pests.

The problem is difficult to manage and prevent. Strawberry cultivars differ in their susceptibility to Type III bronzing, so growers should consider this factor when selecting cultivars for planting. Growing the strawberry crop so as to reduce physiological stress to the plants is a general overall recommendation. Observant growers and PCAs noted that commercial fields that happened to receive insecticide or fungicide sprays prior to a high temperature, high-sunlight intensity bronzing period had in many cases significantly lower bronzing losses compared to adjacent untreated fields. This situation likely occurred because commercial pesticides usually contain additives that protect the products from solar and ultraviolet radiation; such sprays also provided similar protection to the strawberry fruit.