It is at times perhaps tempting to take an uncomplicated view of nutrient deficiency in strawberry.  The mantra goes a little bit like yellow leaves are nitrogen deficient, phosphorous deficiency is given by purple leaves, potassium shortages easily marked by scorched leaves and so on.

I don’t believe any of this is so simple and so attempt to challenge this facile thinking when I have the opportunity to do so.

The following situation was a good one.  A smallish field of ‘Albion’ variety strawberry was not given supplemental fertilizer beyond a standard dose of preplant slow release 18-8-13 in the area of 500 lbs per acre.  The result in late June was as seen below- severely chlorotic plants with a strong tendency towards purpling of the middle age to older leaves.  Additionally, flower production had nearly ceased and fruit was sizing down substantially.

I tested the soil to 6 “down at six different points throughout the field, amalgamated them and got the following results:

Soil pH was a perfectly normal 6.4.  As one can see from the numbers above, plant available nitrogen sources nitrate and ammonium were sort of low (usually want to be 10 ppm for either one), while phosphorous was a quite sufficient 58 ppm.

Tissue samples consisted of leaf blades.  Four separate samples were taken from different sections of the field and each sample consisted of a dozen leaflets of middle age- meaning not the very old ones at the bottom of the plant nor the young tender leaves emerging from the center of the crown. 

Comparing the nitrogen results from the leaf blade with the 2.6-2.8% concentrations described as sufficient as from UC Publication 4098 and Dr. Tim Hartz’s work last year, we see that this vital nutrient is a full percentage point below what is deemed necessary for normal production.  Phosphorous, while below the 0.36% field average taken by Dr. Hartz’s study last year, is still understood to be well above the 0.10 % baseline given by UC Publication 4098.  All other nutrients, with the exception of zinc (15 ppm compared to a recommended 18-20 ppm), in this sample are well within sufficiency ranges.

Why then the pronounced purpling of the leaves if the phosphorous is not dramatically, if at all, insufficient in the tissue?  There are in fact other possibilities.  A lack of nitrogen, which is a component of some amino acids and other compounds, can result in a similar build up of excess carbohydrates as in phosphorous deficiency.   Essentially, both deficiencies have the same result then that these carbohydrates can end up being used in anthocyanin synthesis resulting in accumulation of these flavonoid pigments.  Some are red, others pink and some purple.  This may be an explanation why strawberry leaves lacking in nitrogen but not ostensibly lacking in phosphorous are actually turning red to purple. 

So, while undoubtedly light green to yellow leaves are still a good indication of nitrogen deficiency in strawberry, don't be fooled that the purple that often accompanies it is actually caused by something else like a lack of phosphorous.

Thank you to Soil Control Lab in Watsonville for processing and evaluating these samples.

There have been a few reports as of late concerning a rather abrupt wilting of blackberry plants in certain fields.  Some of these wilts have been found to be caused by raspberry crown borer, which merits a discussion of this pest in this space.

Crown borer is actually simple to diagnose in the field.  One will notice a rather pronounced wilt of the plant, and a subsequent evaluation of the crown of the plant will find a hole there, usually with some sawdust like material deposited outside.  Excavation and opening this section of the plant will find significant tunneling and usually a rather large larva of a half an inch or longer in length, which of course explains the wilt, since most of the water conducting elements are ruined by all this activity and no longer functioning in bringing water higher up to the plant.

As the reader can see from the picture below, crown borer adults look a lot like a wasp, but they are not.  They are a clearwing moth that looks like a wasp to protect it from predation.  Note the long antenna and extensive scales on the individual that distinguish it from a regular wasp in the picture below. 

Briefly, the life cycle of crown borer is two years long.  It appears that the adults are active here in Watsonville in late June and so they would be laying their red-brown eggs on the undersides of leaves around this time as well.  Larvae, after emerging from the eggs somewhere in the area of a month later, work their way down the cane, where they either find a place in the bark to hide or begin excavating a tunnel in the crown.  In either case, the larva is not embedded in the cane yet and rather shallowly situated.  The larva continues to tunnel into the crown through the following year, by the time which the damage can be quite extensive.  Late in the summer, the larva must go into pupation, a state which it apparently remains in until the following spring.   There is unfortunately no degree day model available for raspberry crown borer yet- this would be an exceedingly useful tool, since we are able to use pheromone traps to detect emerging adults and would be able to count with fair accuracy when the susceptible larvae are emerging and working their way down the cane.

A test using drenches at the base of the cane (Johnson and Kim, 2011. The Bramble pp 8-9) found that Brigade, Assail and Entrust all reduced the amount of crown borer in infested caneberries.  Altacor, also registered in caneberries, has also been recommended for crown borer but it is not clear how well this worked in controlling them.  These applications are a function of how closely one is to getting the small, superficially hidden larvae of the first year after they hatch.  The deeply embedded larvae of the second year would be near impossible to get to and kill, by my estimation.

It is still my recommendation that crown borer infested plants be removed immediately from the field and destroyed.  I have yet to see an infestation that is extensive in a field, so plant removal is essentially rouging and keeping the populations of this still rather infrequent pest in check rather than reducing yield by large scale plant removal.  This is important, since blackberry plantations tend to be long lived, and maintaining them as clear as possible of crown borer is a good step in the direction of maintaining long term field viability.

There are pesticides mentioned for management of raspberry crown borers in this article.  Before using any of these products, check with your local Agricultural Commissioner's Office and consult product labels for current status of product registration, restrictions, and use information.

Thank you to Chris Matthews and Doyle Goins for their assistance with the information for this article.

I was taken to several raspberry fields today to investigate an unusual and rather concerning plant disorder.  As one can see from the pictures below, leaves were significantly yellowed, with an abrupt change without regard to leaf venation from yellow to green in many cases.  Some leaves had fallen off, and severely yellowed leaves fell off at the touch. Within the field of yellow, there were many spots of green.  The undersides of the leaf were showing the same, but with the addition of fluffy orange spots throughout. The symptoms were widespread and affected the lower quarter to third of nearly every plant in the visited fields.  Clearly, this was fungal but the question was what fungus?

The literature does admit that downy mildew can occur on raspberries and even result in yellow discoloration.  However, this disease is always marked by purple blotches on the upper side of the leaf, and fluffy mycelial growths on the underside of these very spots.  Microscopic evaluation of several infected leaves today gave no sign of the telltale bunches of mildew mycelia and sporangia masses.  This disease is definitely not downy mildew, since few of the symptoms are consistent with this pathogen.

Rather, and this can be seen from the photos below, the masses of orange to yellow spores on the undersides of infested leaves had were typical of the rusts, and in many more advanced cases one could find the diagnostic teliospores of Phragmidium rubi-idaei, the causal organism of yellow rust.

The extent of leaf yellowing in the pattern depicted below is unusual.  Especially the sharp demarcation of green to yellow.  Short of the defoliation, the literature does not refer to yellowing of the leaf in association with yellow rust infestation.  This bears watching. 

Growers report that these symptoms have appeared very rapidly, and have advanced significantly over the past few days.  Rust spore germination is contingent on water, so that we have had a significant rainstorm followed by mostly cool weather since has a lot to do with the severity of this current epidemic, in my opinion.  

The rusts are controlled by a reduction of moisture.  One way is of course to increase air circulation in the hedgerow, but even better is the use of tunnels which sharply reduce the humidity on and around the plants.  The fungicides Rally and Pristine are registered and very effective against rust.  Organic growers have access to fixed coppers and oils.   All of these materials can reduce fruit yield if used excessively and too early on sensitive developing plant tissue, so growers should not strive to eliminate rust from the hedgerow with repeated fungicide applications, but rather just limit it's spread to the upper foliage where flowering and fruiting is going on.

There are several fungicides mentioned for management of rusts in this article.  Before using any fungicides, check with your local Agricultural Commissioner's Office and consult product labels for current status of product registration, restrictions, and use information.

Starting Friday and over the weekend, a number of area blackberry growers have reported high numbers of lygus bugs in their fields.  I checked on one field this morning, and indeed the level of infestation is astonishing. Somewhere in the area of ten early stage nymphs can fall out from a few shakes at a single spot at any number of points all across the field. This is without precedent in any type of berry and merits some thought and discussion.

The field that was viewed this morning was in full harvest with a lot of fruit in the yet green to pink stage.  Primocane was still growing vegetatively with no flowering.  I do not think lygus presents any harm to the quality of the fruit or the vegetative plant, other than as an incidental contaminant in the baskets of harvested fruit, much along the lines of what we have recently seen with stinkbugs.  Lygus bug feeding damages the flower and developing fruit, so once the druplets have made and the fruit has formed, these bugs will no longer be able to harm it.  Since the field examined today has next to no flowers, the nymphs must be feeding on sap from the newly developing vegetative tissue.  Whether this causes harm to the plants remains to be seen.

Beyond the contamination of harvested cited above, the possibility that this colossal lygus population shifts over to later bearing varieties which are entering into flowering would give some reason for concern.  

Fortunately, blackberry growers have several options available to them to control these pests.  One of the best combinations we found from our work in managing lygus in strawberries has been a pyrethroid + neonicotinoid applied when the lygus are very small as they are now.  Always bear in mind that this powerful combination of pesticides will weigh heavily on beneficials.  Additionally, the neonicotinoids registered in blackberries present some issues with maximum residue limits (MRL's) to fruit exported to Canada.

Organic growers have few options short of border weed removal and vacuuming available to them unfortunately.

There are pesticides mentioned for management of lygus bugs in this article. Before using any of these products, check with your local Agricultural Commissioner's Office and consult product labels for current status of product registration, restrictions, and use information.