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.

There have been now two field closures over the past week in the Watsonville-Salinas berry production district, and the following is meant to make some sense to growers for what to do to minimize losses in the event of a leafroller larva detection.

USDA inspectors are not agriculturalists, nor pest managers for that matter.   We in the industry should be aware that they are bound by a set of rules that at times may seem at odds with sound pest management, but it yet is incumbent upon us as professionals to work with them and come out with the best solution for both parties which is the elimination of leafrollers and re-opening of the field with as little impact to the normal management of the field as possible.  

Growers should be aware that outside of the USDA inspections taking place monthly at each cooler, fruit sent for export out of the USA is being sampled and inspected by county personnel. They have long done this to look for pathogens and other insects and recently this repetoire has been including leafroller larvae.  On finding a suspect larva, the county inspector forwards it to USDA inspectors, who will make an identification of the larva if possible at the cooler and also forward it on to the DNA identification laboratory in Sacramento.  The process from find to absolute positive identification at the DNA lab is about a week, but can take longer in the case of very small larvae or pupae.
It appears that growers are advised of a possible LBAM find on the same day, and a hold is put on the source field.   Inspectors will pay a visit to the hold field to find larvae, which they inevitably do.  Now, the grower has the option of re-routing fruit from the field under the hold order to clients within the quarantine area or destroying it. Both have happened this last week.  It is truly tragic when another market cannot be found for fruit, but such is the nature of this quarantine.  
Once advised of the hold, arrangements are made to have inspectors observe a pesticide application of the infested field with the purported goal of controlling LBAM.  In the interest of time, the process of arranging a pesticide application and field re-inspection is allowed to move forward even in the absence of a full positive identification for LBAM.  Time of initial suspect LBAM find to time of observed spray seems to depend on a number of factors, but generally happens within a week.

In berries, the list of allowed pesticides for inspector observed, mandated sprays is unfortunately rather short.  For strawberry growers, fortunately Intrepid, Entrust and several Bt formulations are included, but notably Coragen, Success and Radiant, which are highly effective and actually have light brown apple moth on their labels along with being pretty soft on beneficials and the surrounding environment, are not allowed.  Instead, we are additionally allowed crude materials such carbaryl (7 day pre-harvest interval, pollinator caution, devastating to beneficials), and chlorpyrifos (21 day pre-harvest interval, seriously harmful to beneficials). A superior type horticultural oil is allowed, but only at a minimum rate of 1% volume to volume, which for a petroleum distillate is pretty risky in sensitive crops like strawberries or caneberries.
Once the approved pesticide application is done, arrangements are made to have the field re-inspected in the interests of confirming that it has been cleared of leafroller larvae and re-opening it.  This is a rather important point for berry growers, because when this happens depends on what material that has been applied.  Based on our experience, inspections have been taking place one day after an application of Entrust, and more than several days after Bt formulations.  If you are willing to take a chance of burning your plants and fruit, the superior type oils also are supposed to give one day. Intrepid and Confirm as insect growth regulators (IGR's) are known to act more slowly so again garner a re-inspection after more than a few days.
It seems in light of all of the above, the strategy that berry growers should be taking in relation to leafroller management should be as per the following:

1- Keep fields clear of leafrollers. Period.
2- In the unfortunate event of a possible LBAM find and hold on the field, spray the field IMMEDIATELY with the very effective and labeled materials such as Coragen or Radiant in strawberries and Delegate in caneberries. By doing so, you are giving these materials time to act in full while you make arrangements for inspections and sprays with the USDA.
3- Make arrangements for mandated spray with the USDA.  
4- Make application of Entrust since you should be able to get inspectors in the next day to confirm that it has cleared the field of leafrollers.  
5- Undergo inspection from USDA to re-open field. Note that your spray of the better material from the day of the hold will now have taken full effect along with the effects of the Entrust, and you should by now have dramatically reduced, if not eliminated, leafroller larvae from your field.

There are pesticides mentioned for management of leafrollers and light brown apple moth 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.