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.

One of the less well understood issues in our industry on the Central Coast is the phenomenon of yellowing of strawberry plants in certain areas of the district, especially in a number of fields north of Salinas.  The following is meant to share what we have found out on this problem so far, and discuss some of my thoughts about the most probable cause.

While there are many causes of yellowing in strawberry plants, for example lack of nitrogen, iron or zinc, the yellowing of strawberry plants in the Salinas area seems to stem from something else and occurs in the same area, year after year.  In fact, some spots no more than a few meters square give the same symptoms every time strawberries are planted there.  Yet, subsequent plantings of other crops such as broccoli or lettuce do not show any yellowing. 

To address the thought that the yellowing comes from nutritional deficiency, I have taken many samples with colleagues of these yellow plants and never found anything exceptional nutrient wise.  Consider the table below which is an eightfold replicated comparison taken in a large strawberry field south of Castroville with large areas of yellow plants in a field of healthy green plants:

What one immediately sees from the table above is that the trend is actually for yellow plants to have HIGHER levels of essential nutrients than their apparently healthier counterparts. 

Interestingly, manganese is very much higher, and a t-test tells us significantly so, in the yellow plants than in green plants.

So, the assumption that nutrient deficiencies are leading to this yellowing of the plants is not backed by the evidence of a plant tissue test.  To be sure, yellowing from nitrogen tends to be stronger on the outer, older leaves as this mobile nutrient is transported to the younger leaves.  Deficiency of zinc generally has a green halo around the leaf edges.  Perhaps the symptoms are consistent with that of iron deficiency, and indeed the iron from the soil sample from around the plant itself is significantly higher around the green plant than the yellow.  However, the levels of iron in the evaluated plants are well above those described as critical by UC Publication 4098 and Tim Hartz’s strawberry fertility work in 2010.

One of the considerations though all of this research is that the yellowing is caused by waterlogging and a subsequent deficiency in the amount of oxygen available to the plant.  This is not necessarily water or saturation that is easily measurable at the surface and may be deeper down in the bed.  It is also possible that salinity, which has a slight inverse effect on the solubility of oxygen in water, is also playing a role. 

Plants respond to decreased oxygen levels, known as hypoxia, in different ways and some species are in fact quite sensitive to this condition.  Roots, as the plant organ which face the hypoxic condition in a waterlogged soil, respond to this stress by switching from respiration to a fermentative metabolism which in turn increases the demand for carbohydrates.   That this metabolic change in strawberry is the cause of the yellowing in our strawberries is something which yet remains to be explored.

As a final thought, consider the the fourth picture below in which the drip tape on the right was clogged and less water delivered to that bed for several weeks.  The result was a lessening in the yellowing of the plants in that bed, and only that bed.  Absolutely, this is not a very scientific evaluation, but it does strongly suggest that excessive water from the plant's perspective has something to do the yellowing we know from around Salinas.

In the past two weeks, growers have been reporting strawberry field situations in which plants are not growing well, are falling behind in size and production, and are showing symptoms of collapse. Initially the older leaves lose their normal bright green color and begin to turn a dull gray green. These leaves later wilt, collapse, and become brown and dry. Early in disease development, the wilting leaves often occur only on one side of the plant. Without exception, the dead and dying foliage is restricted to the outer, older parts of crowns and the inner younger leaves remain symptomless. Examination of the plants showed that roots were normal and not diseased. The internal crown tissue is likewise healthy in appearance and not discolored. In affected fields, symptomatic plants are randomly scattered throughout large sections of the planting.

Plants submitted to the UC Cooperative Extension diagnostic lab (supported in part by the California Strawberry Commission) were tested for a wide range of pathogens. All such samples tested positive for the Verticillium wilt pathogen (Verticillium dahliae) and were negative for Macrophomina, Fusarium, or other pathogens.

The confirmation of Verticillium wilt is, of course, a major concern for growers. Presently the only effective management options are to rotate and plant strawberry in locations that do not have infested soils and/or to fumigate with effective materials. Because V. dahliae can survive in the soil for many years, even in the absence of a plant host, the disease is a long-term concern for growers. A major concern is that as our industry moves away from methyl bromide-chloropicrin pre-plant fumigation, these Verticillium wilt situations may become more common.    

The Verticillium pathogen survives in the soil by producing microscopic, resilient resting structures called microsclerotia. Because microsclerotia are in the soil, growers should be reminded that the movement of significant amounts of infested soil (via mud clinging to equipment and vehicles or by in-field disking) will move the pathogen to uninfested locations. Researchers also find that microsclerotia can be found in high numbers in old strawberry crop residues. Note that the strain of V. dahliae that infects strawberry can infect other plants such as lettuce.

Identification: Angular leaf spot (ALS) of strawberry is a familiar disease to growers and PCAs who are experienced with this crop. It occurs to some extent every season in coastal California. Like most bacterial diseases of crops grown in our region, ALS development and spread is dependent on splashing water; therefore, this disease is typically active only during the winter and early spring months when rains occur in California. Symptoms consist of small (from 1/16 to 1/8 inch wide) spots that first become visible on the lower surfaces of the leaves and have distinctive, straight margins at the edges of the spots (photo 1). Because of these straight edges, the spots have a rectangular or angular shape. Early in disease development the spots appear water-soaked; as disease progresses, the spots turn brown as the leaf tissue becomes necrotic. The upper leaf surfaces will also show the angular spots, with surrounding tissues turning red or yellow (photo 2). The bacteria that cause ALS often ooze to the surface of the lesions, resulting in a sticky film that covers the spot surface (photo 3). As this sticky matrix dries, the exuded bacteria form a crystalline, amber layer.

Spots can merge together, causing much of the leaf tissue to become necrotic and diseased. ALS is readily observed by holding leaves up against sunlight or other light source, which readily highlights the rectangular spots. ALS typically is restricted to the lower, older foliage of the strawberry plants. By the time the rains cease and the plants are vigorously growing, ALS will not occur on newly formed leaves that grow in the summer and fall. ALS does not occur on strawberry petioles, stems, or fruit. On occasion ALS can be observed on the green calyx attached to the developing strawberry fruit.

Situation in 2011: Because of the unusual occurrence of significant rainfall in the months of April and May, ALS is continuing to occur in strawberry fields and is spreading to additional foliage. Therefore, the disease is persisting later into the growing season than usual.

The pathogen: ALS is caused by the pathogenic bacterium Xanthomonas fragariae (Xf). This bacterium has a very narrow host range and only affects strawberry. Likewise, the Xanthomonas pathogens that cause bacterial leaf spot of lettuce (Xanthomonas campestris pv. vitians) and black rot of crucifers (Xanthomonas campestris pv. campestris) will not infect strawberry.

Disease cycle: For production strawberry in central coast California, the initial inoculum usually comes in with the strawberry transplants. However, since Xf can survive in the soil on strawberry plant residues, it is possible for Xf to carry over from one strawberry crop to another if back-to-back plantings are made. Bacteria in the strawberry transplants become active as the plants grow and are spread to leaves and adjacent plants by sprinkler irrigation (used to establish the transplants) and rains.

Impact and management: For production strawberry in California, ALS generally is of low concern. There is no documentation that ALS has any significant lasting effect on strawberry plants, and researchers have not documented significant yield reductions due to this disease. The actual strawberry fruit are not susceptible to infection. Management options are therefore not generally needed and in any case are limited. Use transplants that do not harbor the bacterium. Copper foliar sprays can be applied, but these applications are not very effective and can cause phytotoxic damage to the strawberry plants. Resistant cultivars are not available. In some situations, strawberry plants or fruit (with attached calyces) infected with Xf may be subject to quarantine regulations if being moved out of the state of California.