Is there any biosafety level scale for plant pathogens?
Plant pathogens are not a health hazard to the experimenters. However, they can pose health hazards to plants and cause damage to ecosystem. There are plant biosafety levels: BL1-P up to BL4-P (also sometimes abbreviated as BSL-1P and so on). The four levels have an increasing order of stringency of containment. Containment of plant pathogens (also seeds and pollen) is the primary concern, not exposure to them.

From NIH guidelines:


  For research involving plants, four biosafety levels (BL1-P through
  BL4-P) are described in Appendix L, Physical and Biological
  Containment for Recombinant or Synthetic Nucleic Acid Molecule
  Research Involving Plants.  BL1-P is designed to provide a moderate
  level of containment for experiments for which there is convincing
  biological evidence that precludes the possibility of survival,
  transfer, or dissemination of recombinant or synthetic nucleic acid
  molecules into the environment, or in which there is no recognizable
  and predictable risk to the environment in the event of accidental
  release.  BL2-P is designed to provide a greater level of containment
  for experiments involving plants and certain associated organisms in
  which there is a recognized possibility of survival, transmission, or
  dissemination of recombinant or synthetic nucleic acid molecule
  containing organisms, but the consequence of such an inadvertent
  release has a predictably minimal biological impact.  BL3-P and BL4-P
  describe additional containment conditions for research with plants
  and certain pathogens and other organisms that require special
  containment because of their recognized potential for significant
  detrimental impact on managed or natural ecosystems.  BL1-P relies
  upon accepted scientific practices for conducting research in most
  ordinary greenhouse or growth chamber facilities and incorporates
  accepted procedures for good pest control and cultural practices. 
  BL1-P facilities and procedures provide a modified and protected
  environment for the propagation of plants and microorganisms
  associated with the plants and a degree of containment that adequately
  controls the potential for release of biologically viable plants,
  plant parts, and microorganisms associated with them.  BL2-P and BL3-P
  rely upon accepted scientific practices for conducting research in
  greenhouses with organisms infecting or infesting plants in a manner
  that minimizes or prevents inadvertent contamination of plants within
  or surrounding the greenhouse.  BL4-P describes facilities and
  practices known to provide containment of certain exotic plant
  pathogens
  
   
  
  Section III-D-5-a. BL3-P (Plants) or BL2-P + biological containment is recommended for
  experiments involving most exotic (see Section V-M&dagger;, Footnotes and
  References of Sections I-IV) infectious agents with recognized
  potential for serious detrimental impact on managed or natural
  ecosystems when recombinant or synthetic nucleic acid molecule
  techniques are associated with whole plants.
  
  
  &dagger;Section V-M.  In accordance with accepted scientific and regulatory practices of the discipline of plant pathology, an exotic plant pathogen (e.g., virus, bacteria, or fungus) is one that is unknown to occur within the U.S. (see Section V-G, Footnotes and References of Sections I-IV).  Determination of whether a pathogen has a potential for serious detrimental impact on managed (agricultural, forest, grassland) or natural ecosystems should be made by the Principal Investigator and the Institutional Biosafety Committee, in consultation with scientists knowledgeable of plant diseases, crops, and ecosystems in the geographic area of the research.


These descriptions refer to plant experiments in greenhouses. When working with plant pathogens in a lab (in vitro culture or on whole plants in incubators), similar guidelines should be followed.