Diversity, Molecular Characterization, and Fungicide Sensitivity of Phytophthora ssp. causing damping off blight and root rot in floriculture crops
ANNUAL PROGRESS REPORT - JUNE 2003
Project Title: Diversity, molecular characterization, and fungicide sensitivity of Phytophthora spp. causing damping-off, blight and root rot in floriculture crops
Researcher/Institution Information:
D. Michael Benson, North Carolina State University, Department of Plant Pathology, Raleigh, NC 27695-7629 Phone: 919-515-3966 Fax: 919-515-5657 E-mail: mike_benson@ncsu.edu
Mary Hausbeck, Michigan State University, East Lansing, MI 48824 Phone: 517-355-4534 E-mail: hausbec1@pilot.msu.edu
Jaesoon Hwang, North Carolina State University, Department of Plant Pathology, Raleigh, NC 27695-7629 Phone: 919-515-6828 Fax: 919-515-5657 E-mail: jhwang2@unity.ncsu.edu
Industry Needs
The nature and importance of Phytophthora species attacking floriculture crops in the United States has not been fully explored until recently with work supported by AFE and the USDA SAF Initiative. Phytophthora species have a wide host range on floriculture crops and samples submitted to plant disease clinics often come back as a Phytophthora problem. Although fungicides are used in disease management the effectiveness of these and the possibility of fungicide resistance may pose a serious breakdown in control. In addition, a potential exists for the movement of Phytophthora spp. on infected plant material between growers as material moves from propagator to finisher. Also on-site sources of Phytophthora spp. may be important for initiation of disease outbreaks. By determining the nature and source of Phytophthora spp. attacking floral crops, scientists will be able to help growers focus their disease management efforts where it counts.
Research Summary
Survey and isolate identification. Over 480 Phytophthora isolates were collected in a survey of 29 greenhouse establishments growing 46 floral crops in North Carolina during the past 2 years. The greenhouses surveyed represented small to large operations across the mountain, piedmont, and coastal plain of NC from those with less than $10,000 in annual sales to those with more than $500,000. Between 50 and 75 isolates from a single host at a given greenhouse were collected, where possible. Isolates were characterized based on morphological and PCR methods. Across the collection of isolates 37% were P. cryptogea, 57% were P. nicotianae, 5% were P. palmivora and <0.8% were a possible new species. The PCR method proved very useful in identification of Phytophthora isolates to species, particularly in the case of P. cryptogea because this species is often mistakenly identified as P. drechlersi as morphological characters are similar in many respects. In most cases only a single species of Phytophthora was found on a given crop at a given greenhouse. Isolates of P. cryptogea were collected from dusty miller and gerbera. Isolates of P. nicotianae were found on African violet, lavender, pansy, petunia, and vinca, while P. palmivora was collected from infected ivy. It is interesting to note that we did not find P. drechlersi a species that has been reported on poinsettia in New York and Michigan over the past few years. Additional analysis of isolates for genetic diversity was done by AFLP analysis in the laboratory of Dr. Mary Hausbeck at Michigan State University to compare North Carolina isolates of P. nicotianae with isolates from floriculture crops in other states.
Mating type. Phenotypic characterization of isolates for mating type and fungicide sensitivity was completed for the isolates this year. Tester isolates of P. cryptogea and P. nicotianae were used in pairings with collected isolates to determine whether the isolate was A1 or A2 mating type. All isolates of P. cryptogea were A1 mating type, while 268 isolates of P. nicotianae were A2 from African violet, lavender, petunia, and vinca, and eight isolates were A1 all from pansy. The occurrence of both mating types of P. nicotianae in North Carolina could result in greater pathogen variation through sexual reproduction in greenhouses where crops with both mating types are grown.
Fungicide sensitivity. Mefenoxam (Subdue Maxx) is one of the most common fungicides used for control of Phytophthora diseases on floriculture crops. Since mefenoxam is a systemic, fungistatic compound, the potential exists for development of pathogen insensitivity to the compound with repeated use. Collected isolates were screened for insensitivity to mefenoxam by incorporating 1 or 100 ppm into an agar medium and comparing isolate growth at these concentrations with growth of the isolate in agar without mefenoxam. Normally, sensitive isolates are completely inhibited by 1 ppm of mefenoxam. Up to 20% of P. nicotianae isolates were insensitive to mefenoxam i.e. they grew at 100 ppm. Only 8% of the P. cryptogea isolates were sensitive to mefenoxam, while 19% were intermediate and 73% of P. cryptogea isolates were not inhibited by even 100 ppm of mefenoxam. This is alarming as the results suggest that growers with P. cryptogea or P. nicotianae in their crops would not expect disease control with mefenoxam. In such a situation alternative fungicides must be found.
Dissemination of Results
Hwang, J., and Benson, D. M. 2003. Identification and characterization of Phytophthora species from floriculture crops in North Carolina. Phytopathology (Supplement) (APS Annual meeting poster)
Hwang, J., and Benson, D. M. 2003. Identification, mefenoxam sensitivity, and mating type of Phytophthora spp. attacking floriculture crops in North Carolina. USDA ARS SAF Initiative, Raleigh.
Lamour, K. H., Daughtrey, M. L., Benson, D. M., Hwang, J., and Hausbeck, M. K. 2003. Etiology of Phytophthora drechsleri and P. nicotianae (= P. parasitica) affecting floriculture crops. Plant Disease (accepted).
