- Posted By: Mark Bolda
- Written by: Mark Bolda
We have had the misfortune of discovering Armillaria root rot on blackberry on the Central Coast. This is significant in that root rots such as those from species of Phythophthora or Verticillium are extremely rare in blackberries cultivated in this area.
Generally known as a pathogen of forest trees and orchard crops such as apples and avocados, this root rot is known by the common name “oak root fungus” and is caused by several different species of Armillaria fungus. The above ground symptoms of Armillaria root rot are those common to other root rots, that is to say wilt and stunting of canes and leaves (Photo 1 below). Below ground, roots in blackberry show obvious signs of rot frequently accompanied with whitish mycelial growth (Photo 2 below). While the literature also says this mycelium forms sheets and fans that was not the case in this particular situation. Importantly, the symptomatic plants are found in patches which slowly advance outward over time. In more advanced infections, one would also find black rootlike strands of 3-4 mm in diameter known as rhizomorphs. In the situation addressed here on the Central Coast, the infected areas were limited to rows and tended not to cross over into the next, so the paucity of root growth and moisture in the area between rows may be limiting to disease spread from row to row.
Armillaria infections of blackberry are a result of previously infected orchard plantings. Since this pathogen can persist for decades in infected stumps and other woody material, the history of a field should be of serious consideration for blackberry growers when looking at new ground to plant.
While it is undoubtedly a good idea to fumigate a piece of ground known to have been infected with Armillaria fungus, fumigation itself might not be enough because even the best fumigant may be hard put to penetrate infested stumps and other woody debris. With this in mind, to the extent possible, remove woody debris from a cleared orchard or forest prior to fumigation.
Once an infection is underway, the only way to address the problem is to limit the spread of the disease. This means removing symptomatic plants and as much of the roots as possible along with those plants immediately adjacent. No, this will not eradicate the fungus from the plantation but will serve to at least limit its spread through the field to other plants.
- Posted By: Mark Bolda
- Written by: Mark Bolda
The recent implementation of an eradication program and proposed quarantine of grapes of a small area in south Santa Cruz County due to multiple finds of European grapevine moth, Lobesia botrana, puts us in the position of having to discuss in detail the possibility that this pest could occur in blackberry.
While blackberries are described in the literature as having been a host of European grapevine moth and thus will fall within compliance actions, I have reason to doubt that European grapevine moth will be found in our cultivated blackberry varieties here on the Central Coast because of the following:
1- The clustered inflorescences of grapes are quite distinct from those of blackberries, yet have been described as a major part of the attraction of grapes to European grapevine moth. Blackberries occur in loosely formed inflorescences of less than 10 fruit, and are not in compare to the numerous, closely grouped fruit of wine and table grapes. Simply put, the morphology of the inflorescences of cultivated blackberries in our area is not amenable to European grapevine moth.
2- The species of blackberries on which European grapevine moth was reported in UC and CDFA literature is given as Rubus fruticosus, which is the wild blackberry commonly known as Himalayan blackberry found alongside roads and in the woods. Cultivated American blackberry cultivars do not come from this R. fruticosus, and rather come from R. allegheniensis, R. argutus, R. frondosus, R. ursinus, R. macropetalus and R. loganobaccus.
3- The current campaign outside of species specific pheromone based mating disruption underway by area blackberry growers in eradicating the other invasive leafroller species, the light brown apple moth, from their fields will go a long, long way in keeping fields clear of European grapevine moth should it ever find its way there.
The link for the full provisional guidelines is here:
http://www.ipm.ucdavis.edu/EXOTIC/eurograpevinemoth.html
In spite of of the factors predicting very low odds of European grapevine moth infesting Central Coast blackberries, growers are nonetheless cautioned to be vigilant and are encouraged to contact this office should they have questions concerning this matter.
- Posted By: Mark Bolda
- Written by: Mark Bolda and Steven Koike
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.
- Posted By: Mark Bolda
- Written by: Mark Bolda
I had a fruitful meeting with PPQ Supervisor Rick McKay, Inspector Leah Gayagas and currently serving Santa Cruz County Agricultural Commissioner Mary Lou Nicoletti on May 19. This meeting was concerning the current inspection and field closure procedures for light brown apple moth in strawberry and caneberry growers in the quarantine zone which covers all of Santa Cruz county and most of the berry production areas of North Monterey counties.
At the time of the meeting, three raspberry fields and one strawberry fields had been closed because of positive finds, so this matter is of considerable urgency to the berry growing community.
Inspections are to take place for each grower every thirty days, either at the cooler for those sending fruit there, or at the field for those growers who are sending fruit out of the quarantine zone venues such as farmer's markets and fruit stands. In addition to this, it is worth reminding growers who are growing fruit for export out of the USA that county phytosanitary inspections are also being done on every load, and they will forward suspect leafrollers to the USDA PPQ program.
Inspections are being done as before, that is to say that 2% of a designated load are evaluated for leafroller presence. If a leafroller is found in the load- and this means any leafroller species - a hold is put on the load and also on the field from where the load came. If no leafrollers are found, the field is clear for another thirty days. A suspect leafroller gets shipped up to the identification laboratory in Sacramento immediately via FedEx and a positive or negative determination should be available within one or two days of the find. If determined to be positive, the hold on the fruit and field continues, if negative then the hold on the fruit and field is lifted. As noted before however, if the larva is very small or is in the pupal stage, the determination will take longer, maybe up to a week and a half.
Field closure is done after an inspection of the field from where the infested load originated. Size of the closure is not arbitrary, it depends on where the larvae are. If they are in a corner of the field only, then only that corner will be closed. If they are spread evenly through the field, then the whole field will unfortunately be closed. Growers should be aware of the trend of finding more leafrollers along the edges of fields in wooded and riparian areas.
One key point out of this meeting for berry growers and their consultants is that once a hold is put on a field, it must be inspected for light brown apple moth larvae before being released. The date of inspection is negotiable, and it might not necessarily be in one's best interest to have a field inspected immediately, since if there are no larvae found, then the hold on the field is lifted and no regulated spray and subsequent field inspection necessary. So, it would seem a good thing for growers experiencing a load hold and subsequent field hold to get that field cleaned up right away. Arrangements can generally be made for inspectors to come out a few days after a hold is put in place, which would give the grower time to spray a fast acting material like a spinosyn and run a crew through and pick out leaf rolls. By doing all of this effectively, the grower avoids the burden of dealing with the regulated spray (and it's disappointingly short list of allowed chemicals) and follow up inspections. By moving quickly, the grower can avoid all that.
The attachment below is the USDA inspection protocol for fruit inspected at a cooler.
Mark LBAM090
- 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.