- Posted By: Mark Bolda
- Written by: Steven Koike, Plant Pathology Farm Advisor
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.
Readers of this blog should know that there is a new site concerned with strawberry research topics throughout California:
Topics covered currently are lygus and the cultivation of strawberries in sterile substrate. Have a look.
Since we have had our first field closure in strawberry as of last night, it is important for us all to review how the USDA regulatory process unfolds for growers here.
Field closure is expensive and to be avoided. For readers who are not growers or familiar with commercial strawberry production, consider that the average weekly take out of a field can be 400 boxes per acre that we will conservatively price at 8 dollars a box back to the grower. So the weekly cost of LBAM closure to a fifty acre field is 160,000 dollars. Personally speaking, this enormous penalty is totally out of proportion with the infraction of having a quarantined pest in one’s field, but it is not my decision to make.
Cooler Inspection: USDA inspectors are to visit area coolers at regular intervals, my understanding is that because of the reduced budget for the program they will be checking in at each local facility once a month. This may change however depending on the vagaries of Federal budgets and politics. Once at the cooler, inspectors take a subsample from a load of fruit from each production field for that day. The inspectors are VERY thorough, the field closure from last night came from a larva tucked under the calyx. Unfortunately, such a larva often cannot be identified right away and so a hold is put on fruit coming out of that field. In plain English, that means the field is closed and you can no longer harvest and send fruit out of that field.
Field Inspection: Once a field has been identified as possibly infested with light brown apple moth, the next step is for inspectors to do an inspection of the field itself. I have seen field inspection, and it does not leave one leaf unturned. For somebody with experience, leaf rolls are easy to see, and inspectors walk with up to six people abreast, one person per row. Since this is their job, day in and day out, they are really good at it and if you have a leafroller, it will be found.
Treatment of Infested Fields: If no leafrollers are discovered during the field inspection, the field is opened back up. If a leafroller is discovered and furthermore found to be positive for LBAM, the grower is mandated to make an application. He or she chooses from a list of allowed materials, and fortunately this year, according to program director Rick McKay, surfactants and adjuvants are allowed and highly recommended. All parts of the application are observed by inspectors from loading, mixing and the actual spray. Nothing says that the grower can't be making applications before and after this regulated application, but they need to see the one they mandate. Then, depending on the pesticide used, inspectors return after a specified number of days to re-inspect the field. If no more leafrollers are found, the field is opened back up. If a leafroller is found, the field remains closed and the application procedure is repeated. Experience from last year says more often than not it takes more than one spray to re-open a field. In some cases, especially in organic fields, which have a much narrower selection of effective materials, it can take more than a month to re-open.
Discussion: It is imperative that growers pay attention to leafrollers in their fields. Yes, they are around, because I have been getting phone calls about this all week long now. Conventional growers have a wide range of materials at their disposal, organic growers less. It is not a bad thing yet to be putting out the pheromone mating disruption twist ties, since we are probably looking at a flight of adult moths in late June, with a subsequent larval infestation in July and August again. In light of the devastating costs of field closure, it might not be too much to have crews go through the fields regularly and be removing rolls.
This is a revision from the previous post:
A couple of samples handled by this office from strawberry over the past two weeks have turned out to be a leafroller species which might not be light brown apple moth. Probably the reason people have been asking about these is the general concern about having leafrollers in one’s production field at all. This is understandable, considering the current regulatory environment which mandates closure of at least part of a field on discovery of one species of leafroller; the light brown apple moth. Fields can also be temporarily shut down as regulators work on distinguishing leafrollers from one another in order to get a positive light brown apple moth identification.
The following short description will be about garden tortrix in strawberry.
This very important for those making a definitive identification in the field. While the UC IPM guidelines refer to garden tortrix as having a "spot on the back of the head" this spot is actually a darkened prothoracic shield behind the head on not on it (see fourth picture below). I would very much like to get a definitive sample from our area of garden tortrix for a clear picture for this blog, so will ask my readership to bring any suspect samples. It would be very helpful.