Leptosillia pistaciae (Voglmayr et al.) Voglmayr, comb. nov. 2019


California Pest Rating Proposal for

Leptosillia pistaciae (Voglmayr et al.) Voglmayr, comb. nov. 2019
Pest Rating: B

Comment Period: CLOSED


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Posted by ka

3 thoughts on “Leptosillia pistaciae (Voglmayr et al.) Voglmayr, comb. nov. 2019”

  1. March 25, 2020 Comment from Faith T. Campbell:
    The California Department of Food and Agriculture has posted for comment a draft Pest Ranking Proposal addressing the pathogen Leptosillia pistaciae. The Department’s draft pest ranking assigns the highest Economic Impact score – three – to this pathogen. It assigns a medium Environmental Impact score – two. The overall ranking is proposed as “B”.

    The Center for Invasive Species Prevention (CISP) believes the Department’s draft pest ranking seriously understates the potential threat that Leptosillia pistaciae poses to California’s native flora and ecosystems. We will provide details in reference to specific points in the draft pest ranking – but the overall picture is important. The entire risk assessment process is so focused on verified information that it fails to explore possible additional consequences. The result is that the official response probably trails behind the actual invasion and will fail to contain it.

    Detailed comments on the specifics sections of the ranking proposal:

    1) Initiating event
    The Pest Ranking Proposal makes no mention that the San Elijo lagoon is not adjacent to any port (it is 27 miles from San Diego, 91 miles from Long Beach). There are human settlements nearby. We are unaware of any waste handling facilities in the area. CISP concludes that it is unlikely that this site is actually the first outbreak of the pathogen. There is no information indicating official surveys of nearby horticultural plantings incorporating species in the Anacardiaceae family. Or surveys of wild plants in the family (of which there are many – see below under point 2).

    2) History and Status
    We note that Leptosillia pistaciae was identified only in 2018. This means that the actual host range is almost certainly unknown. To date, two species in the family have been identified. However, the family includes several important horticultural species, including pistachio, cashew, mango, and smoke trees (Cotinus spp.); and several species of sumacs. California’s wild or naturalized flora includes several possible hosts, among them other native sumacs: R. ovata, R. aromatica, and Malosma laurina (CNPS; full citation below); the ubiquitous poison oaks (Toxicodendron spp.) and the widespread invasive plant genus Schinus. The Pest Ranking Proposal makes no mention of these potential hosts nor does it factor them into the risk assessment.

    Further under this section, the Pest Ranking Proposal mentions movement of greenwaste and nursery stock as potential vectors by which the pathogen might be spread. The Pest Ranking Proposal does not indicate that any effort has been made to assess these important vectors; it does not even mention the possibility of additional hosts, as we outlined above. The Pest Ranking Proposal does note the possible threat from asymptomatic and latent infections.

    Continuing, the Pest Ranking Proposal addresses possible consequences of the pathogen’s establishment. This assessment is inadequate because it focuses exclusively on the two known hosts (rather than recognizing the potential additional hosts) and on spread through airborne spores (not expanding the reference above to movement of diseased material as greenwaste or horticultural stock).

    The assessment of consequences falls short also in evaluating the environment impact that would arise from harm to only one of the potential hosts. CISP certainly agrees that the pathogen could reduce biodiversity, disrupt natural communities, or change ecosystem processes. However, surely these impacts will be more severe if additional wild shrubs, e.g., the other sumacs and poison oaks, are killed by Leptosillia pistaciae.

    CISP is particularly concerned that the assessment claims there is no uncertainty in the overall Pest Ranking Proposal. To the contrary, we have identified numerous fundamental areas of uncertainty, including
    • Extent of established outbreak – and its actual initial site
    • Potential host range
    • Associated ecosystems and geographic areas at risk
    • Potential presence of Leptosillia pistaciae in nursery stock throughout the state.

    Therefore CISP urges the CDFA to carry out a new risk assessment that addresses explicitly these unknowns. We anticipate that the result will be a significantly higher ranking for environmental impacts.

    Thank you for considering our views. We look forward to working with the California Department of Food and Agriculture to identify and implement measures to maximize protection of California’s native flora from this newly detected threat.

    Submitted by Faith T. Campbell

    President
    Center for Invasive Species Prevention

    Sources:

    Rhus and related species native to California: California Native Plant Society
    https://calscape.org/loc-california/Rhus(all)/vw-list/np-1?

    Rhus species used in horticultural plantings in California: CalFlora
    https://www.calflora.org//cgi-bin/specieslist.cgi?where-genus=Rhus

    Response to Faith T. Campbell :
    The Center for Invasive Species Prevention (CISP) believes the Department’s draft pest ranking seriously understates the potential threat that Leptosillia pistaciae poses to California’s native flora and ecosystems. Each specific comment will be answered below:

    Comment: The Pest Ranking Proposal makes no mention that the San Elijo lagoon is not adjacent to any port (it is 27 miles from San Diego, 91 miles from Long Beach). There are human settlements nearby. We are unaware of any waste handling facilities in the area. CISP concludes that it is unlikely that this site is actually the first outbreak of the pathogen. There is no information indicating official surveys of nearby horticultural plantings incorporating species in the Anacardiaceae family. Or surveys of wild plants in the family (of which there are many – see below under point 2).

    Response: These particular plants were installed in the 1990s and records no longer exist as to where they were sourced, and we do not know when they were infected. We are unaware of other reports of outbreaks near this location or elsewhere in the state. We do not have evidence that other members of the family Anacardiaceae are hosts of this pathogen. Also, we have not seen evidence that this problem has spread beyond its original location (i.e. other Rhus in the nature preserve). It is also notable that it has not been reported in Rhus or any Anacardiaceous plant in native plant nurseries or commercial nurseries.

    Comment: We note that Leptosillia pistaciae was identified only in 2018. This means that the actual host range is almost certainly unknown. To date, two species in the family have been identified. However, the family includes several important horticultural species, including pistachio, cashew, mango, and smoke trees (Cotinus spp.); and several species of sumacs. California’s wild or naturalized flora includes several possible hosts, among them other native sumacs: R. ovata, R. aromatica, and Malosma laurina (CNPS; full citation below); the ubiquitous poison oaks (Toxicodendron spp.) and the widespread invasive plant genus Schinus. The Pest Ranking Proposal makes no mention of these potential hosts nor does it factor them into the risk assessment.

    Response: Although the host range may increase over time, it should not be assumed that all members of a plant family will necessarily be hosts for any given pathogen. Pathogenicity tests have not yet been done to prove that the symptoms observed on the Rhus integrifolia were definitively caused by L. pistaciae. This fungus is closely related to others that are known to be symptomless endophytes. This species, (formerly Liberomyces now Leptosillia) is described as an outlier in its genus such that it is a pathogen of a living trees. The authors of the one paper that describe L. pistaciae as a pathogen (Vitale, et al. MycoKeys 40:29–51). caution that their pistachios may only have become diseased as the trees were stressed. They suggest that detailed studies are necessary to evaluate the influence of stress on the virulence of L. pistaciae. As of now in California, L. pistache has only been isolated in association with the dieback of 5 Rhus shrubs. For pathogens that seem to be activated by stress, it is hard to know if they are part of the endophytic flora until someone takes it on as a research project.

    Comment: Further under this section, the Pest Ranking Proposal mentions movement of greenwaste and nursery stock as potential vectors by which the pathogen might be spread. The Pest Ranking Proposal does not indicate that any effort has been made to assess these important vectors; it does not even mention the possibility of additional hosts, as we outlined above. The Pest Ranking Proposal does note the possible threat from asymptomatic and latent infections.

    Response: Nursery stock can be a pathway for the movement of pathogens. California nursery inspectors are trained to recognize disease symptoms on woody plants and submit samples to the CDFA labs. To date, no samples of L. pistaciae have been received. Green waste is occasionally a pathway for plant diseases, but this pathway is usually a concern for pathogens which form long lasting survival structures such as chamydospores or sclerotia. No such spore stages have been identified for this pathogen. Although there may be a possibility of additional hosts, none have yet been demonstrated.

    Comment: Continuing, the Pest Ranking Proposal addresses possible consequences of the pathogen’s establishment. This assessment is inadequate because it focuses exclusively on the two known hosts (rather than recognizing the potential additional hosts) and on spread through airborne spores (not expanding the reference above to movement of diseased material as greenwaste or horticultural stock).

    Response: This pest rating will cover the pathogen whenever it is found, even if the host is not named in this rating proposal. Although it is presumed to spread by airborne spores based on reports from similar fungi, no epidemiological work has been published on short or long-distance spread of L. pistaciae.

    Comment: The assessment of consequences falls short also in evaluating the environment impact that would arise from harm to only one of the potential hosts. CISP certainly agrees that the pathogen could reduce biodiversity, disrupt natural communities, or change ecosystem processes. However, surely these impacts will be more severe if additional wild shrubs, e.g., the other sumacs and poison oaks, are killed by Leptosillia pistaciae.
    CISP is particularly concerned that the assessment claims there is no uncertainty in the overall Pest Ranking Proposal. To the contrary, we have identified numerous fundamental areas of uncertainty, including
    • Extent of established outbreak – and its actual initial site
    • Potential host range
    • Associated ecosystems and geographic areas at risk
    • Potential presence of Leptosillia pistaciae in nursery stock throughout the state.

    Response: This disease would have a greater environmental impact if the host range was expanded, however, we are unaware of hosts beyond the two described in this pest rating proposal and we have no information on which to propose adding any additional hosts. There is always some uncertainty when new pests are found, but the pest rating can only be based on what is known now. We have added to the uncertainty section following your suggestion and have reposted the proposal for an additional 45 days. If we learn more about how this pathogen behaves under CA conditions, and if there is evidence that it is more widely distributed, this may affect the rating and the rating can be changed.

    The “B”-rating is assigned for pests of the agricultural industry or environment which score medium to high and which are of limited distribution in the State of California. Authorized mitigating regulatory actions: Plants and plant products found infested or infected with or exposed to a “B”-rated pest may be subject to immediate quarantine actions listed under subsections 3162(e). This allows the State or the County Agricultural Commissioners to take action to eradicate the pathogen in their counties.

    After considering the Pest Risk Proposal and the comments, the following rating is recommended: Rating: B

    1. COMMENT: From
      Themis J. Michailides, PhD
      Plant Pathologist – Fruit & Nut Crops and Mycotoxins
      University of California-Davis
      Kearney Agricultural Research & Extension Center

      Hi Heather,

      To answer simply I think Leptosillia will have no effect against pistachio. Regarding threat to Rhus integrifolia, because it was isolated from these plants it does not mean it is a pathogen. .
      Because the fungus was isolated from dying or dead Rhus plants we cannot really assume automatically that this is the cause and imposes a threat. Someone should perform Koch’s postulates that this fungus is the pathogen. There can be many other factors that might have contributed to killing of Rhus, and the Leptosillia (an endophytic fungus) was able to grow and dominate. This is a very slow growing fungus! I have my justified reservations about this being a serious pathogen, if it were a pathogen..

      We are currently studying other more serious diseases, like anthracnose (Colletotrichum spp.) that we discovered in some pistachio in Sacramento Valley causing a disastrous disease on pistachios. This anthracnose disease killed 75% of the Australian pistachio crop in 2010 when they had excessive rains before and during harvest there that year..
      Before characterizing this fungus as a threat to pistachio or even to Rhus spp. plants, we need to do some inoculations of pistachio under controlled conditions (in the greenhouse) with currently commercial pistachio cultivars and see if this fungus is able to attack these pistachios and/or Rhus bushes (if they can be grown in the greenhouse under controlled conditions). The Sicilian variety where the fungus was found to cause small (round restricted cankers) is very different cultivar from the cultivars we grow. I also need to know under what conditions this Rhus plants grow. I am not familiar with it, plus need to see photos of the “canker” before providing any suggestions,

      I have been working on pistachio for more than 30 years now and I dealt with both foliar and fruit blights and also canker diseases of pistachio. I never isolated from cankers of pistachio this particular fungus. I know this fungus has been reported in Sicilian pistachios, but even there they showed that this fungus is not very aggressive but it is very slowly growing on media. There is no concern of any presence of this fungus in our pistachios. There are more aggressive pathogens that attack our pistachios (such as Botryosphaeriaceae canker fungi) and that is a concern of the growers who have pistachios, especially now when the Botryosphaeriaceae diseases have spread on many forest trees and agricultural crops and they are very aggressive now occurring on walnuts, almonds and pistachios. Chaparral plants are very susceptible to Bot diseases, especially after stress. I wonder if those Rhus bushes were killed by Botryosphaeriaceae and then Leptosillia developed….

      The fungus Leptosillia pistaciae (old name Liberomyces pistaciae) is considered as an endophyte, that probably needs stress conditions in order to attack and cause diseases. We do have pistachios planted in salty soils and they are under severe stress sometimes., We never had any problems with this Leptosillia pistaciae fungus, and I have monitoring pistachio diseases since 1985.

      1. Reply to Dr. Michailides

        Thank you for your comments. Given your expertise in Pistachio pathology in California, it is reassuring that you don’t think this fungus will be a problem.

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