Raspberries, blackberries, blueberries and strawberries aren't the whole story of berries on the Central Coast.  There are many other species to be found around the farms and forests of our area.

The photos below are of thimbleberries, a species of Rubus found in the forests of the Santa Cruz mountains.  This set of photos was taken in the lower reaches of Nisene Marks State Park.

Thimbleberries, Rubus parviflorus, are commonly found growing in the woods next to clearings in humus rich soil.  The plants can be quite tall and over one's head in height.  The foliage has a maple-leaf shape, is velvety and is rather large at some six to ten inches across on a fully grown plant.  The canes are not thorny, and fruit is borne sparsely in clusters of three to five on second year canes.  

The fruit of the thimbleberry are each composed of multiple druplets, and are quite soft when ripe so I doubt they would  ship very well.  They aren't super tasty.   At any rate, the season is underway right now, and will probably continue for another two weeks or so in Santa Cruz County.

Two weeks I was called out to investigate the situation in organically farmed red raspberry that the reader can see below.  The plants are pushing forth an impressive crop of fruit and overall the plant stand is strong yet numerous laterals are showing a yellowing of the leaves, especially towards the tips. 

The Problem: The youngest leaves on the ends of fruiting laterals were showing a distinctive yellowing.   Looking closely at affected leaves, one can see that the veins of the affected leaves remain green to some extent.

Field Evaluation:  The farm manager and I initially looked around for arthropods (insects and mites) or damage as well as extirpating a few plants to examine the roots and generate samples for submission to the UCCE diagnostic laboratory in Salinas.

We then took 4 replicated samples of yellow leaves and then four replicated samples of green leaves from the same stage (between the 5^th and 7^th leaf from the tip) of apparently healthy laterals.  We also took soil samples down to about six inches deep from four distinct areas of the field.  All samples were submitted to the Soil Control Laboratory in Watsonville for analysis.

Results:

No arthropods of any consequence were found during our visit, nor did the UCCE diagnostic laboratory come up with any plant pathogens.  The mycelial threads we found on the crown and roots (last photo below) of the cane are those of a saprophytic fungus and present no threat to the plant.

The means of the replicated tissue samples were compared through a Student’s t-test and the results are presented below in Table One.

Table One.  Average Mineral Concentration of Green and Yellow Leaves

As one can see, there are several minerals, being magnesium, manganese, calcium and iron, which are significantly lower (p<0.05) in concentration in the yellow leaves than in the green.  Conversely, potassium is much higher in the yellow than in the green.

The unfortunate thing is that since we don’t have published guidelines for these sorts of raspberry varieties  (for example like the recently published DRIS study in strawberry), we have to refer to out of state raspberry fertility guidelines for other varieties to get a handle on the meaning of all these numbers.  Using these guidelines, we find that the concentration of manganese in the yellow leaves falls below the consensus of a lower threshold of sufficiency of about 30 ppm.  In contrast, concentrations of iron, calcium, and magnesium, even though they are significantly lower in the yellow leaves than the green, are still within the generally accepted ranges of sufficiency.

We do get a glimpse also of the tolerances of these raspberry plants to chloride and sodium, which can be useful for future reference.  The green leaves contain about 5000 ppm chloride and around 170 ppm sodium.  These concentrations in my experience would be cause for some plant distress in strawberry, but apparently these levels are fine here.

The averages of the soil samples are below. As there was no area in the field showing more yellow than others, simply four composites of six individual samples were taken to get an understanding of the background mineral concentration of the soil.

Table Two.  Average Mineral Concentrations of Soil

Nothing jumps out here from this table of soil concentrations.  Nitrates might be a tad lean at 10 ppm, phosphorous is typically high, pH is normal, ECe is a comfortable 0.91 and the micronutrients are available in some quantity.

The question is then what is the conclusion?  We have no arthropod or pathogen compromising the plants ability to take up nutrients or anything else.  Lacking any other explanation, my take would be twofold.  One is that the pH of 7.6 in the soil is limiting the manganese, and that the big fruit load could be also drawing off this nutrient from the leaves and moving them to the fruit.  My choice of corrective action would be to add manganese, along with iron and magnesium, just to be sure, to this planting.

Thanks to Patrick Kingston and his boss for having me out on this call.  It’s always great to collaborate on issues with such enthusiastic and smart up and comers in our industry.

A very important concept for all pest managers to understand is that the smaller the target insect is, the higher the ratio of surface area of its body becomes in comparison to its mass.  Since most insecticides are applied to the surface of the insect, this theoretically means that one is able to deliver a higher dose of insecticide per unit weight of the smaller insect than the larger and subsequently control the smaller more readily.

To demonstrate, let us approach the problem mathematically.

Let us take a lygus nymph compared to a lygus adult. To simply the math, we assume both bug stages are completely spherical:

Surface area small nymph (1 mm radius):  12.6 sq mm

Volume small nymph: 4.2 cubic mm

Surface area adult (3 mm radius): 113 sq mm

Volume adult: 113 cubic mm

Now let us examine what happens when we spray each insect with a thin sheet of water of thickness 0.001 millimeter and consisting of 1% poison and which perfectly covers the surface area of both the lygus nymph and adult.

So, continuing with our calculations, the amount poison applied to the small nymph:  

0.001 mm thickness of water sheet  x 12.6 sq mm surface area = 0.0126 cubic mm water x 1% poison = 0.000126 cubic mm poison to the small nymph.

Divide the cubic mm volume poison by the volume of the small nymph:

0.000126 cubic mm poison/4.2 cubic mm = 0.00003 cubic mm poison per cubic mm pest

Amount poison applied to the adult:

0.001 mm thickness water x 113 sq mm surface area = 0.113 cubic mm water x 1% poison = 0.00113 cubic mm poison to the adult (10 times the poison as the small nymph!)

Divide the cubic mm volume poison by the volume of the adult:

0.00113 cubic mm poison/113 cubic mm = 0.00001 cubic mm poison per cubic mm pest.

Let us express cubic mm as milligrams (mg) as is often done (1 cubic mm water = 1 mg = 1 ml) to simplify the concluding explanation:

So, the 0.00003 mg poison per mg pest for the small nymph is 3 times the amount of 0.00001 mg poison per mg pest of the adult.  

This is all fine and good, but now let us move from the mathematical theory to the reality of actual pesticide application.  To begin, please consider the two photos provided by Ed Show which include strips of paper  approximating lygus of two basic sizes, the longer being the 3/8" of an adult and the smaller approximating the size of a lygus nymph.

One can see very readily from this experiment that the problem with the mathematical theory is that of the assumption of perfect coverage of the lygus surface area with a thin sheet of water in my calculations, rather than using sprayed spaced droplets one can see in the second photo below.  Clearly, the wider surface area has a higher probability of catching droplets than the smaller area, and the further apart the droplets tend to be, the lower the probability becomes of the smaller target getting hit with them.  As a matter of fact, the droplets could be so far apart so as to even miss completely the smaller nymphal area, while delivering at least something of a dose to the adult.

With very, very closely spaced droplets, the greater the importance the surface area to mass ratio, but with droplets spaced more apart from one another the greater the importance of the surface area alone.

This comparison of the ideal and the practical argues heavily in favor of using lots of water carrier to create an abundance spray droplets, especially when dealing with a pest of small size.  Water delivered in low gallonages and big droplets from low pressure delivery certainly packs more punch per droplet, but if it doesn't hit the target it will serve little purpose.  

This is a reminder that UCCE in Monterey County is holding a major meeting concerning mites in strawberries this coming June 27.  Spanish translation will be offered.

Link to the agenda here:

http://cemonterey.ucanr.edu/files/166685.pdf

This is yet another extension meeting not to be missed.