Evaluation of seed treatments for organic vegetable production in the Pacific Northwest

This study addresses management of damping-off in organic vegetables in central Washington, using pea as an early season, model crop. The disease is being examined under low soil temperature and high soil moisture conditions of early spring in the Columbia Basin, when damping-off is most severe. PhD student, Ana Vida Alcala, spent 2009 and spring 2010 in Pullman, completing courses. In 2009, soil samples were collected from 37 organic fields in the Basin. Alcala obtained >300 Pythium isolates from the soils, which she identified to species using DNA sequencing. For each of 19 Pythium species, up to 9 isolates from across the Basin were evaluated in growth chamber trials for pathogenicity on pea in cool, moist, organic soil from a grower-cooperator’s field. Isolates of 11 of the 19 species were pathogenic on pea. In 2011, organic seed/drench treatments (microbial and non-microbial) were evaluated in four field trials: 15 treatments in each of 2 trials in grower-cooperator, organic pea crops in the Basin; and 20 treatments in inoculated pea and sweet corn trials at the WSU Mount Vernon NWREC. The more promising treatments were evaluated again in 2012 applied to both primed and non-primed seed in each of two field trials. In addition, the use of primed seed that was dried with biochar instead of dry air and then treated with Nordox (organic copper), showed potential for improving pea emergence, height, and yields in one of two grower-cooperator trials in the Columbia Basin. Results from the 2011 and 2012 field trials were presented at the Pacific Northwest Vegetable Assoc. Convention & Trade Show in November of 2011 and 2012. The 2011 trial results also were published in Plant Disease Management Reports (Vol. 6:ST011 and ST012), and the 2012 trial results were submitted in Dec. 2012 for publication in Vol. 7. Finally, Alcala is evaluating the potential use of electrolyte leakage measurement of pea seed lots as a predictor of the risk of damping-off at different levels of inoculum pressure of Pythium spp. in organic soil, which growers may be able to use in combination with soil assays for this pathogen to select appropriate seed lots for reduced risk of damping-off. The growth chamber trials completed in 2012 on this aspect of the project are very promising. Alcala anticipates completing her PhD by May 2013.

Grant Information

  • Project ID: 045
  • Project Status: Complete


  • Principal Investigator(s): du Toit, L.
  • Investigator(s): Paulitz, T., Porter, L.
  • Student(s): Alcala, A. V.
  • Grant Amount: $40,000
  • 2008 Progress Report (PDF)


  • Principal Investigator(s): du Toit, L.
  • Investigator(s): Paulitz, T., Porter, L.
  • Student(s): Alcala, A. V.
  • Grant Amount: $40,000
  • 2009 Progress Report (PDF)


  • Principal Investigator(s): du Toit, L.
  • Investigator(s): Paulitz, T., Porter, L.
  • Student(s): Alcala, A. V.
  • Grant Amount: $78,677


  • Principal Investigator(s): du Toit, L.
  • Investigator(s): Paulitz, T., Porter, L.
  • Student(s): Alcala, A. V.
  • Grant Amount: $35,306


  • Principal Investigator(s): du Toit, L.
  • Investigator(s): Paulitz, T., Porter, L.
  • Student(s): Alcala, A. V.
  • 2013 Progress Report (PDF)


Alcala, A.C., Paulitz, T.C., Schroeder, K.L., Porter, L.D., Derie, M.L., and du Toit, L.J. 2016. Pythium species associated with damping-off of pea in certified organic fields in the Columbia Basin of central Washington. Plant Disease 100:916-925.

Alcala, A.C., du Toit, L.J., Derie, M.L., Holmes, B., Coffman, G., Gatch, E.W., and Porter, L.D. 2013.
Evaluation of priming and Nordox seed treatment for controlling damping‐off in organic pea
crops in central Washington, 2012. Plant Disease Management Reports 7:ST002.

Alcala, A.C., Porter, L.D., Derie, M.L., Holmes, B., Coffman, G., and du Toit, L.J. 2013. Evaluation of
seed treatments and priming for controlling damping‐off in organic pea crops in the semi‐arid
Columbia Basin and maritime Skagit Valley of Washington, 2012. Plant Disease Management
Reports 7:ST001.

Alcala, A.C., Derie, M.L., Holmes, B., Gatch, E.W., Porter, L.D., Coffman, G., and du Toit, L.J. 2012.
Evaluation of organic seed and drench treatments for controlling damping‐off in pea and sweet
corn in Mount Vernon, WA, 2011. Plant Disease Management Reports 6:ST011.

Alcala, A.C., Derie, M.L., Holmes, B., Gatch, E.W., Porter, L.D., Coffman, G., and du Toit, L.J. 2012.
Evaluation of organic seed and drench treatments for controlling damping‐off in organic pea crops
in the semi‐arid Columbia Basin of Oregon and Washington, 2011. Plant Disease Management
Reports 6:ST012.

Alcala, A. C., Paulitz, T. C., Porter, L. D., and du Toit, L. J. 2011. Profile of Pythium spp. in certified
organic fields for vegetable production in central Washington. American Phytopathological
Society Annual Meeting, Honolulu, Hawaii, 6‐10 August 2011. Phytopathology 101:S4. (Poster

Alcala, A.C., and du Toit, L.J. 2009. Management of damping‐off in organic vegetable crops in the
Pacific Northwest. Sustaining the Pacific Northwest 7(4):5‐7.

Lindsey du Toit’s Vegetable Seed Pathology team website, with a section on Avi Alcala’s project:


Short‐Term:  This project has resulted in a greater understanding of the primary Pythium spp.
that appear to cause damping‐off in cool, early spring conditions in certified organic fields in the
Columbia Basin in which pea crops are grown. This research revealed the diversity of Pythium
spp.,  pathogenic  and  non‐pathogenic,  in  certified  organic fields,  and the  prevalence  of the
pathogenic  species  in  relation  to  non‐pathogenic  species.  In  addition,  the  field  trials
demonstrated that almost all of the products currently registered for use in certified organic pea
crops for damping‐off control are either not effective at reducing damping‐off in cool, spring
conditions in central and western Washington, or are inconsistent. Therefore, the use of these
seed treatments by growers is not warranted, which corroborates comments received during an
informal survey of organic growers in 2009 regarding their use of organic seed treatments. The
2012 field trials demonstrated that combining organic seed treatments with primed seed did not
enhance  efficacy  of these treatments.  In fact, this  had  negative  effects  on  pea  emergence,
growth,  and  yield.  However, the  use  of  biochar  instead  of  forced  air to  dry  primed seed,
particularly when combined with Nordox, an organic cuprous oxide, improved pea emergence,
height, and yield in one of two grower‐cooperator trials in the Columbia Basin in 2012, and
warrants further investigation (potential intermediate‐term impact).

Intermediate‐Term:  Results of the pea seed lot EC testing and the soil baiting for Pythium spp.
pathogenic  on  pea  in  cool,  moist  soil  conditions  appear  promising  for  helping  growers
determine the potential risk of damping‐off in their fields by measuring: i) the EC value of the
seed lots they plant, and ii) inoculum levels of the three primary pathogenic Pythium spp. in
fields they are considering planting. The real‐time PCR assays investigated for the three main
pathogenic Pythium species provide a means of determining inoculum levels in growers’ fields.
However, a major limitation of using real‐time PCR assays for testing soils for target pathogens is
the very limited amount of soil that can be tested (up to 5 g/sample, with potentially no more
than 10 samples tested/field in order for the test to be viable economically), which may need to
represent fields as large as 150 acres. However, even if the use of real‐time PCR assays does not
prove  viable  for  testing  grower’s  fields,  this  research  demonstrated  increasing  growers’
awareness of the specific EC measurement of seed lots they will plant will enable the growers to
make appropriate decisions based on the relative risk of damping‐off for their seed lots. Seed
lots with higher EC values can potentially be reserved for planting in fields with a history of no or
low risk of damping‐off, or for later planting when warmer soil temperatures mean a reduced
risk of damping‐off. However, there is a limit to the latter choice as most growers double‐crop
organic pea and sweet corn crops, which means pea crops cannot be planted too late; and early
planting  of  pea  crops reduces the  difficulty  of  organic  control  of  nightshade,  a  weed that
produces toxic berries that are very difficult to separate from peas in processing plants.

Long‐Term:  The potential practical  and economic  aspects of priming  and drying (with  air or
biochar) the large volumes of pea seed that are planted in fields in the Columbia Basin each
spring needs to be assessed. Through education and outreach, this research project should result
in increased awareness among organic pea growers of the value of knowing the quality of
their  pea seed  lots, measured  using  EC  values,  and  how to  use this  information for spring
planting decisions (within the boundaries of processor contracts and other practical aspects of
relatively  large‐scale,  organic  production  of  pea  crops).  Real‐time  PCR  assays  for the three
primary pathogenic Pythium species causing damping‐off in early spring planting conditions in
the Columbia could potentially be offered as a diagnostic service by private labs (e.g., Western
Labs in Idaho), to use in combination with pea seed lot EC measurement, to determine which
fields to plant with which pea seed lots.