California Pest Rating for
Thekopsora minima P. Syd. & Syd. 1915
Pest Rating: C
PEST RATING PROFILE
On May 2, 2017, a shipment of blueberry (Vaccinium corymbosum) plants showing symptoms of rust were intercepted in San Francisco by San Francisco County Agricultural Officers. The shipment had originated in Oregon and was destined to a wholesale garden store in San Francisco. A sample of symptomatic leaves was collected by the County and sent to the CDFA Plant Pathology Lab for diagnosis. On May 22, 2017, Suzanne Latham, CDFA plant pathologist, identified the fungal pathogen associated with the diseased leaf tissue as Thekopsora minima. The pathogen was assigned a temporary Q rating. Subsequently, the consequences of introduction and establishment of T. minima in California are assessed and a permanent rating is proposed herein.
History & Status:
Background: Thekopsora minima is a fungal pathogen that causes rust disease in blueberries, cranberries, rhododendrons, and other plants in the Ericaceae family. The pathogen completes its life cycle on two different hosts (heteroecious), namely, blueberries and hemlock, and rust disease can lead to extensive defoliation of severely infected plants.
The blueberry leaf rust pathogen was first recorded as endemic in Northeastern America and Japan. During the past decade, it was introduced on infested Vaccinium corymbosum to other countries including South Africa, Mexico, Australia and Colombia (EPPO, 2016). In the USA, it has been reported mainly from northeastern states and, more recently, from the Western Pacific states of Oregon and California (Wiseman et al., 2016; Shands et al., 2017).
Prior to 1993, taxonomically, Thekopsora minima was generally accepted as a member of a species complex known as Pucciniastrum vaccinii, which was considered the causal agent of blueberry rust. However, Sato et al., (1993) identified three distinct rust fungi species on Vaccinium spp., of which one of them, namely, T. minina, is pathogenic on blueberry, while the other two species, Naohidemyces vaccinii (formerly P. vaccinii) and N. fujisanensis, were not regarded as pathogens of blueberry, although they infected other Vaccinium species. Sato et al., (1993) also noted that at that time, T. minima, occurred, in eastern North America and Japan. Nevertheless, because of the past taxonomic confusion of the species complex, the true global distribution of T. minima may be uncertain as some records attributed to Pucciniastrum vaccinii in Argentina, Hawaii (USA), and Spain may be misidentifications of T. minima (Schrader & Maier, 2015). Thekopsora minima is also known by its synonyms: Peridermium peckii Thüm, 1880, Uredo minima Schwein, 1922, and Pucciniastrum minimum (Schwein.) Arthur 1906 (Farr & Rossman, 2017).
In California, Naohidemyces vaccinii has been reported on Vaccinium membranaceum (thin leaf huckleberry), V. caespitosum (dwarf bilberry), V. parvifolium (red huckleberry), V. ovatum (California huckleberry), and Vaccinium sp. (French, 1989). However, recent reports from several states in the US (Oregon and Michigan), China, Mexico, and South Africa, have indicated that Thekopsora minima is the primary pathogen on northern and southern highbush blueberries (Rebollar-Alviter et al., 2011; Shilder & Miles, 2011; Wideman et al., 2016; Zheng et al., 2017). Rust symptoms have been occasionally observed on various southern highbush blueberry cultivars (Vaccinium corymbosum) within California’s central coastal area, with particular incidences noted in Santa Barbara County in 2010 and 2006 (personal communications: Dr. Timothy D. Miles, Assistant Professor of Plant Pathology, California State University Monterey Bay, and Dr. Janet C. Broome, Global Plant Healthy Senior Manager, Driscoll’s, 2017). Rust in blueberry was also observed in Ventura County, and has most likely been in the State since the early 2000s (personal communication: Dr. Janet C. Broome, Driscoll’s, 2017). In 2016-2017, rust symptoms, observed on several blueberry plants and cultivars in a field trials in Watsonville, Santa Cruz County, were confirmed by molecular sequencing to be caused by T. minima and marked a first published report of this pathogen in California (Shands, et al., 2017). On August 9, 2017, in order to officially substantiate the presence of blueberry rust in California, official samples of symptomatic blueberry plant tissue were collected from infected plants in Santa Cruz and Ventura Counties, by the respective County Agricultural officials and submitted to the CDFA Plant Pathology Laboratory for identification of the associated pathogen. Following morphological and molecular sequence analysis, Cheryl Blomquist, CDFA plant pathologist, confirmed the pathogen to be T. minima.
Disease development: Teliospores of T. minima hibernate on blueberry leaves on the ground and after germination in late spring, infest the alternate host, Tsuga spp., via basidiospores. Aeciospores are produced and infest Vaccinium and other Ericaceae host plants resulting in the production of urediniospores. The latter ensure disease spread within the crop during the entire growing season. Furthermore, it has been shown that other closely related blueberry rust species are capable of surviving as mycelium in plant buds and directly producing urediniospores in spring, thereby eliminating the need of the alternate host (EPPO, 2016). It is not known if this is the case for T. minima in California where two native host species, Tsuga heterophylla and T. mertensiana can serve as alternate hosts for the pathogen to complete its lifecycle. These two species are also native to the Pacific western states although the fungus has not been recovered from Tsuga (Wiseman et al., 2016; Shands, et al., 2017). The other two hemlocks that are alternate hosts, T. canadensis and T. diversifolia, are not generally cultivated in California but may be present in small areas of private production and nurseries. Pfister et al., 2004, experimentally determined the predicted optimum temperature for urediniospores to be 19.5°C, with a 5% variation in uredinia production between 17.5 and 22°C.
Dispersal and spread: Spores of Thekospora minima are spread over short distances to nearby plants by wind and rain. Spores may also be spread by human contact, clothing, equipment and packaging. Long distance spread occurs mainly through passage of infected plants including fruit to non-infected regions (EPPO, 2016, Tasmania, 2014).
Hosts: The uredinial and telial stages of the pathogen are found on the main hosts in Vaccinium spp., namely, V. angustifolium var. laevifolium (lowbush blueberry), V. corymbosum (highbush blueberry), V. membranaceum (deciduous huckleberry) and V. erythrocarpum (southern mountain cranberry) in the family Ericaceae. Other hosts belong to different genera in the same family: Azalea sp., A. pontica var. daviesii, Gaylussacia sp., G. baccata (black huckleberry), Leucothoe sp., Lyonia nezikii, L. ovalifolia var. elliptica, Menziesia sp., Pernettya sp., Pieris sp., Rhododendron nudiflorum, R. ponticum, and Rhodora canadensis. The aecial stage of the pathogen is found on the alternate host, Tsuga spp., (hemlock; Pinaceae), T. canadensis (eastern hemlock), T. diversifolia (Japanese hemlock), T. heterophylla (western hemlock), T. mertensiana (mountain hemlock) (EPPO, 2016; Farr & Rossman, 2017; Wiseman et al., 2016).
Symptoms: Initial symptoms appear as small yellow, chlorotic leaf spots on upper surfaces of young leaves. As infection progresses these lesions turn rust or brown-colored, enlarge and coalesce covering large areas of a leaf. On the underside of leaves, small flecks surrounded by water-soaked halos develop turning into yellow-orange, powdery pustules containing uredinia with urediniospores. Pustules may also develop on blueberry fruit. In severe infections premature leaf drop and plant defoliation can occur and result in decline in fruit yield and flower production (EPPO, 2016).
Damage Potential: Blueberry rust disease caused by Thekopsora minima may result in plant defoliation and decline in fruit and flower production. Generally, under conditions of high humidity required for rust fungi infection, significant losses in blueberry production and other Ericaceae host plants can be expected. However, in California, such high humidity climates are not anticipated in blueberry cultivated regions and T. minima has not caused significant rust disease in blueberry, even though it has been in the State for over 17 years ((personal communication: Dr. Janet C. Broome, Driscoll’s, 2017). Infected plants do not suffer from leaf drop, which is generally associated with the rust, and the pathogen has not been an issue of concern for blueberry growers to warrant administration of control measures. Some rust disease is apparent on leaves from early spring into summer following periods of significant rain, however, it is difficult to find infected plants later in the season (personal communications: Dr. Janet C. Broome, Driscoll’s, 2017 and Dr. Timothy D. Miles, CSUMB). Similarly, economic damage to other environmental host plants is expected to be minimal as the pathogen has already been in California for several years without any significant increase of its impact.
Worldwide Distribution: Asia: China, Japan; Africa: South Africa; Europe: Netherlands (restricted distribution), Portugal (present, few occurrences); North America: Canada, Mexico, USA; South America: Colombia; Oceania: Australia (New South Wales, Queensland, Victoria) (EPPO, 2016, 2017; Mostert et al., 2010; Zheng et al., 2017).
In Europe, the pathogen is currently regarded as “Transient, under eradication” in Belgium and Germany (EPPO, 2017). In the USA, it has been reported from Delaware, Massachusetts, Michigan, New York and Oregon (EPPO, 2017; Sato et al., 1993; Schilder & Miles, 2011; Wiseman et al., 2016).
Official Control: Thekopsora minima has been on the EPPO Alert List for the European Union since 2016 (EPPO, 2017). Presently, Thekoposora minima is on the ‘Harmful Organism List” for Peru (USDA PCIT, 2017).
California Distribution: Thekopsora minima has officially been detected in Santa Cruz and Ventura Counties. The pathogen has also been reported from Santa Barbara County (Shands et al., 2017).
California Interceptions: Thekopsora minima has only been detected once in a shipment of blueberry plants intercepted in San Francisco in 2017 (see “Initiating Event”).
The risk Thekopsora minima would pose to California is evaluated below.
Consequences of Introduction:
1) Climate/Host Interaction: Main hosts of Thekopsora minima are in the family Ericaceae and include blueberries, rhododendrons and azaleas. Blueberries are grown in northern coastal and southern coastal regions and in the San Joaquin Valley. Rhododendrons, azaleas and other horticultural hosts are grown throughout California particularly in coastal climates. However, because T. minima requires high humidity for infection and development in order to cause significant disease, it would only be likely to establish in very limited areas of the State. The pathogen is already established in several coastal areas, for the past several years, and rust disease appears typically only during early spring to summer following significantly wet periods.
Evaluate if the pest would have suitable hosts and climate to establish in California.
– Low (1) Not likely to establish in California; or likely to establish in very limited areas.
– Medium (2) may be able to establish in a larger but limited part of California.
– High (3) likely to establish a widespread distribution in California.
2) Known Pest Host Range: Thekopsora minima has a moderate host range. Main hosts of the pathogen are in the family Ericaceae and include blueberries, rhododendrons, and azaleas.
Evaluate the host range of the pest.
– Low (1) has a very limited host range.
– Medium (2) has a moderate host range.
– High (3) has a wide host range.
3) Pest Dispersal Potential: Urediniospores are produced in abundance and ensure disease spread within the crop during the entire growing season. Spores are spread over short distances to nearby plants by wind and rain and may also be spread by human contact, clothing, equipment and packaging. Long distance spread occurs mainly through passage of infected plants including fruit to non-infected regions.
Evaluate the natural and artificial dispersal potential of the pest.
– Low (1) does not have high reproductive or dispersal potential.
– Medium (2) has either high reproductive or dispersal potential.
– High (3) has both high reproduction and dispersal potential.
4) Economic Impact: While blueberry rust disease has been reported to cause plant defoliation, this has not been the case in California, even though the fungus has been present in the State for several years. Blueberry growers have noted that some rust disease is apparent on blueberry leaves from early spring into summer following periods of significant rain, however, it is difficult to find infected plants later in the season. Infected plants do not suffer from leaf drop, which is generally associated with the rust, and the pathogen has not been an issue of concern for blueberry growers to warrant administration of control measures. No yield loss due to this rust pathogen in California has been observed or reported (see; ‘Damage Potential’).
Evaluate the economic impact of the pest to California using the criteria below.
Economic Impact: None
A. The pest could lower crop yield.
B. The pest could lower crop value (includes increasing crop production costs).
C. The pest could trigger the loss of markets (includes quarantines).
D. The pest could negatively change normal cultural practices.
E. The pest can vector, or is vectored, by another pestiferous organism.
F. The organism is injurious or poisonous to agriculturally important animals.
G. The organism can interfere with the delivery or supply of water for agricultural uses.
Economic Impact Score: 1
– Low (1) causes 0 or 1 of these impacts.
– Medium (2) causes 2 of these impacts.
– High (3) causes 3 or more of these impacts.
5) Environmental Impact: Although, horticultural and environmental plants, such as azaleas and rhododendrons, are hosts of Thekopsora minima (see: ‘Hosts’), the pathogen has not increased in its spread or impact in cultivated communities over the past several years of its presence in California. Therefore, no significant impact on the environment or home/ornamental plantings is expected.
Evaluate the environmental impact of the pest on California using the criteria below.
Environmental Impact: None
A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.
B. The pest could directly affect threatened or endangered species.
C. The pest could impact threatened or endangered species by disrupting critical habitats.
D. The pest could trigger additional official or private treatment programs.
E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.
Environmental Impact Score: 1
– Low (1) causes none of the above to occur.
– Medium (2) causes one of the above to occur.
– High (3) causes two or more of the above to occur.
Consequences of Introduction to California for Thekopsora minima: Low (8)
Add up the total score and include it here.
–Low = 5-8 points
-Medium = 9-12 points
-High = 13-15 points
6) Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included.
Evaluation is Low. Thekopsora minima has officially been detected only in few coastal counties in California.
-Not established (0) Pest never detected in California, or known only from incursions.
–Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).
-Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.
-High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.
7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: (Score)
Final Score: Score of Consequences of Introduction – Score of Post Entry Distribution and Survey Information = 7
It is not known if the pathogen will infect hemlock (Tsuga spp.) in California, to complete its life cycle. The pathogen was not recovered from hemlock in California and Oregon (Pacific coastal regions). Hemlock species are widespread in California.
Conclusion and Rating Justification:
Based on the evidence provided above the proposed rating for Thekopsora minima is C.
Calflora. 2017. Information on California plants for education, research and conservation. [web application]. 2017. Berkeley, California. The Calflora Database [a non-profit organization]. http://www.calflora.org/
EPPO. 2017. Thekopsora minima (THEKMI). EPPO Global Database (last updated: 2017-05-19). https://gd.eppo.int/taxon/THEKMI/distribution.
French, A.M. 1989. California Plant Disease Host Index. California Department of Food and Agriculture, Sacramento (Updated online version by T. Tidwell, May 2, 2017).
Mostert L., W. Bester, T. Jensen, S. Coertze, A. van Hoorn, J. Le Roux, E. Retief, A. Wood, and M C. Aime. 2010. First report of leaf rust of blueberry caused by Thekopsora minima on Vaccinium corymbosum in the Western Cape, South Africa. Plant Disease 95: 478.
Pfister, S. E., S. Halik, and D. R. Bergdahl. 2004. Effect of temperature on Thekopsora minima urediniospores and uredinia. Plant disease, 88: 359-362.
Rebollar-Alviter, A., A. M. Minnis, L. J. Dixon, L. A. Castlebury, M. R. Ramirez-Mendoza, H. V. Silva-Rojas, and G. Valdovinos-Ponce. 2011. First report of leaf rust of blueberry caused by Thekopsora minima in Mexico. Plant Disease 95: 772.
Sato, S., K. Katsuya, and Y. Hiratsuka. 1993. Morphology, taxonomy and nomenclature of Tsuga-Ericaceae rusts. Transactions of the Mycological Society of Japan 34: 47-62.
Schilder, A. M. C., and T. D. Miles. 2011. First report of blueberry leaf rust caused by Thekopsora minima on Vaccinium corymbosum in Michigan. Plant Disease, 95: 768. https://doi.org/10.1094/PDIS-12-10-0884.
Schrader, G., and W. Maier. 2015. Express – PRA for Thekopsora minima occurrence. Julius Kühn-Institute, Institute for Plant Health. Translated by Elke Vogt-Amdt. http://pflanzengesundheit.jki.bund.de/dokumente/upload/fee0d_thekopsora-minima_express-pra.pdf
Shands, A. C., T. Ho, and T. D. Miles. 2017. First report of leaf rust on southern highbush blueberry caused by Thekopsora minima in California. Plant Disease (Accepted for publication).
Tasmania. 2014. Blueberry rust (Thekopsora minima P. Syd & Syd). Biosecurity Tasmania Fact Sheet, current as at October 2014. http://www.dpipwe.tas.gov.au/biosecurity/plant-biosecurity/pests-and-diseases.
USDA PCIT. 2017. USDA Phytosanitary Certificate Issuance & Tracking System. Retrieved May 31, 2017. 6:30:49 pm CDT. https://pcit.aphis.usda.gov/PExD/faces/ReportHarmOrgs.jsp.
Wiseman, M. S., M. I. Gordon, M. L. Putnam. 2016. First report of leaf rust caused by Thekopsora minima on Northern highbush blueberry in Oregon. Plant Disease 100: 1949.
Zheng, X., G. Tang, Y. Tian, X. Huang, X. Chang, H. Chen, H. Yang, S. Zhang, and G. Gong. 2017. First report of leaf rust of blueberry caused by Thekopsora minima in China. Plant Disease 101: 835. https://doi.org/10.1094/PDIS-09-16-1379-PDN
John J. Chitambar, Primary Plant Pathologist/Nematologist, California Department of Food and Agriculture, 3294 Meadowview Road, Sacramento, CA 95832. Phone: 916-262-1110, plant.health[@]cdfa.ca.gov.
You must be registered and logged in to post a comment. If you have registered and have not received the registration confirmation, please contact us at plant.health[@]cdfa.ca.gov.
Pest Rating: C
Posted by ls
4 thoughts on “Thekopsora minima P. Syd. & Syd. 1915”
Dear Dr. Chitambar,
Thank you for the opportunity to comment on the proposed pest rating for thekopsora minima (“blueberry rust”). I am the Executive Director of the California Blueberry Commission (“Commission”) an entity of state government enacted to ensure the continuous supply of blueberries in California. One of the purposes of the Commission is to establish a research program to develop an integrated approach to manage pests and diseases common to blueberries.
The Commission agrees with comments submitted by Dr. Broome of Driscolls that blueberry rust has not been a significant issue for the California blueberry industry. We note that one of the Economic impacts considered is the potential that blueberry rust could lower crop yields. This has not been demonstrated despite evidence that the disease has been in California as early as 2000. Moreover, there has not been an increase in blueberry rust over the course of the past 17 years which would suggest that it will not pose a significant threat to the environment.
A proposed “A “rating, however, would have very real and negative economic impacts on the California blueberry industry. The authorized mitigating regulatory actions could involve rejection, quarantine or destruction (see 3 CCR section 3162) all at great expense to the grower. Moreover, California is one of the largest exporters of blueberries in the United States and the Commission is currently in trade negotiations with seven countries. Blueberry rust has not been identified as a pest of concern but to now elevate it to a level “A” rating could create trade barriers in these markets. Given the evidence that blueberry rust has been identified in California on several occasions over the past two decades, yet has not become fully established or destructive, it would appear that the assumptions in the Economic and Environmental Impact section could be re-evaluated and hopefully result in a lower pest rating.
The Commission is committed to working with the Department to research this disease and develop a full understanding of the potential impacts both internal to California growers as well as phytosanitary issues that could impact export markets. Thank you again for the opportunity to submit comments and please contact me with any additional questions or concerns the Department may have.
Alexander J. Ott, Executive Director, California Blueberry Commission, 2565 Alluvial Avenue, Suite 182, Clovis, CA 93611; (559) 221-1800; AOtt@calapple.org
Dear Dr. Ott,
Thank you for the comments from the Commission. They will definitely be taken into account in the re-evaluation of the blueberry rust pathogen prior to the designation of its permanent rating. The commitment and cooperation of the California Blueberry Commission is truly appreciated in this matter.
Dear Dr. Chitambar,
I am writing to you to comment on the proposed pest rating of an “A” for Thekopsora minima, obligate pathogen that causes blueberry rust. Currently it is a “Q” while CDFA considers what permanent rating to give it.
Specifically I wish to comment on item 6) Post Entry Distribution and Survey Information:
As you note in your Pest Rating Proposal http://blogs.cdfa.ca.gov/Section3162/?p=3783, T. minima is a member of a species complex known as Pucciniastrum vaccinii and was considered the causal agent of blueberry leaf rust until Sato et al. (1993) identified three unique species. While Naohidemyces vaccinii has been reported on Vaccinium membranaceum (thin leaf huckleberry) Douglas ex Torr. on Vaccinium caespitosum Michx. (dwarf bilberry), and on Vaccinium ovatum Pursh (California Huckleberry) in CA – See CDFA’s French’s host list records attached ‐ recent reports of T. minima in Australia, China, Mexico, South Africa, and in several states in the US (Oregon and Michigan), indicate T. minima is the primary pathogen on northern and southern highbush blueberries (Wiseman et al., 2016, Schilder and Miles, 2011).
I am attaching a recent Disease Note submitted to the journal Plant Disease, where Dr. Tim Miles, his student, and our research scientist Dr. Thien Ho, report that we have identified what we pathologists believe to be T. minima in California as early as in 2010. “Leaf rust symptoms have been occasionally observed on various southern highbush blueberry cultivars (Vaccinium corymbosum L.) within the CA central coast area with a particular siting noted in 2010 in Carpinteria, CA in an organic field (Mansun Kong, Driscoll’s, personal comm.)”
In addition, our first blueberry breeder, Dr. Brian Caster, believes that the rust has been in California even earlier, since the early 2000s, when southern highbush blueberry varieties started being planted in California, and it has never been seen to cause any disease warranting our growers taking control measures.
More recently, our Oxnard growers recollect that at least since 2015, if not earlier, they saw rust symptoms in blueberries, specifically in the Snowchaser variety. In March 2017, our test plot crew saw some blueberry rust symptoms on a few plants covered with a wind screen. None of those plants suffered from any leaf drop associated with the rust, and symptoms were minimal across the test plot.
In the Watsonville area, in September 2016, blueberry rust symptoms were observed at our breeding test plot grown without tunnels. Species identification using molecular methods was not successful. Rust was observed again at the same location in April 2017, and molecular sequencing confirmed it was caused by the fungus Thekopsora minima. (Aidan C. Shands, Thien Ho, Timothy D. Miles. 2017. First Report of Leaf Rust on Southern Highbush Blueberry caused by Thekopsora minima in California, submitted May 2017 to Plant Disease journal). Further scouting identified blueberry rust at a nearby commercial ranch on Casserly road, occurring mostly at the edge of tunnels on numerous public varieties, including Snowchaser and Emerald.
On August 9, at your and Dr. Cheryl Blomquist’s recommendations, we assisted county staff in Santa Cruz and Ventura Counties to collect and send samples to CDFA for identification.
Finally, we have also observed blueberry rust in Baja California, Mexico. As in CA, rust in Baja California did not appear to cause any significant damage.
I would like to suggest that its distribution in California, in Oxnard and Watsonville, is more like a – Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region), or a Medium (-2) ‐ Pest is widespread in California but not fully established.
Then related to our experience with the rust being here since maybe as early as 2000, as I outlined above, I would like to comment on 4) Economic Impact: While blueberry rust disease has been reported to cause plant defoliation, we have not seen this over the years that we have seen rust in California on our plants. It shows up, causes some rust on leaves infected early in the spring into summer, but by later in the season it is hard to find. We have not seen yield loss due to this rust. The very real economic impact related to possible quarantine status of the pest is quite important to us as a company, so this concern is very real. However, since we have seen a rust in California for many years without major increase in the disease, I think the solution to this challenge for us, and for other companies in California who hope to export planting stock, will be to learn more about the disease here in California and thus through understanding it potentially changes in other countries’ phytosanitary status for blueberry rust may be worked out. We hope to work with you and the California Blueberry Commission and others on this issue.
Then on the questions of 5) Environmental Impact: I would also suggest that since we have not seen the rust increasing in our farmed environments over the past 17 years, that this should be taken into account in estimating what might happen to native plants and the potential need for any kind of additional control programs.
Finally, I wish to comment on 1. Climate/Host Interaction: Various suitable hosts are present in California, as you note, however, rusts in general tend to require a fairly high humidity to infect and cause significant disease, and based on the Mediterranean climate of California, we do not anticipate the climate being as conducive as it might be in Florida, Central Mexico, or more humid parts of Australia. We believe since it has been here since the early 2000s, that it is more like a “Low ‐ 1” and likely to occur in limited areas and to a limited extent, as we have seen to date.
In summary, from our diagnostic records, direct observation, and our conversations with growers and test plot crews, we have strong evidence that blueberry rust caused by the fungus Thekopsora minima is a disease that has been already established for several years in California coastal areas and into Mexico. Our experience to date is that the symptoms are relatively mild and the fungus does not appear to cause significant damage to blueberry plants. Our growers have not felt themselves to be negatively impacted. We do not think this disease should be a significant concern for the blueberry industry.
I would like to ask you respectfully to consider recalculating your pest rating based on this new information.
We welcome a chance to discuss this with you further. I greatly appreciate that you have the best interests of the State and our blueberry growers in mind in making this determination.
If you have any questions please do not hesitate to contact me. We look forward to working with you on this process.
Janet C. Broome, Ph.D. Global Plant Health Sr. Manager Watsonville, CA. 95076,
Attachments (sent via email) – 1) Accepted with revisions research article, 2)MS Excel file extract of blueberry diseases diagnosed by CDFA as part of French’s Host Listing
Dear Dr. Broome,
Thank you for your comments. You have presented some recent and valuable information for consideration. Thank you for assisting with the collection and submission of samples to the CDFA Lab. The pest rating for Thekopsora minima will be re-evaluated prior to the designation of its permanent rating. Thank you.
Comments are closed.