Organic Farm Calls: White Rot on Garlic, Leek, & Onion

This week a central Jersey grower of organic produce calls to report problems in a field of overwinter garlic.

The grower reports there is no history of previous allium crop production in the field. Two different varieties are being grown. On inspection, one variety shows yellow, stunted and wilting plants in small patches in the field; the other variety is vigorous and symptom free.

What might be the cause of this problem? What steps do you take to find the definitive answer? What can our organic grower do to avoid further crop loss?

What might be the cause of this problem? 

With allium crops – Garlic, Leek, and Onion – we think of four problems in this scenario:

Photo by Bruce Watt,
University of Maine

• Onion maggot 
• Overwinter weather injury followed by soft rot bacterial infection 
• Pink root
• Onion white rot, Sclerotium cepivorum 

Further inspection shows: no maggot damage (it’s too early), nor pink root. We look for evidence of overwinter injury followed by soft rot bacteria invasion, either from rough conditions or from tender less well-adapted varieties. Some bacterial soft rot is present. But, in this field yellow wilting plants are found in small groups; a pattern not consistent with typical winter injury. We pulled 4-6 weak plants and half had mats of white mycelia on the bulbs.

Sabrina Tirpak

What steps give the definitive answer?

We take samples to the Rutgers Plant Diagnostic Lab, where Sabrina Tirpak puts them under the microscope, giving us the diagnosis we suspected: black spherical bodies (sclerotia) of Sclerotium cepivorum are seen.

 

What does our grower need to do?

Immediate roguing and removal of diseased plants should be carried out, including removal of neighboring plants showing symptoms. Sclerotia can live for 2 decades in soil, waiting for next long term rotation of susceptible onion crops. Root exudates specific to Allium spp. from the next crop stimulate the sclerotia to germinate and invade the crop. Long-term rotations are a must, but not a complete solution.

 

Discussion – Prevention & Treatment

How do white rot sclerotia get there? There were no previous allium crops. As one variety did not show symptoms, sclerotia may have been introduced with the planting stock. I am not aware whether sclerotia live on wild onion. On any farm, but particularly in organic production avoid introducing onion white rot sclerotia to the farm in contaminated planting stock. If white rot is introduced, long rotations out of allium crops, and not moving equipment through affected fields to new onion fields is advised.

 

Treatment of onion white rot in organic production fields. Management of onion white rot fills books. Our Vegetable Pathology Specialist, Andy Wyenandt recommends Contans WG, an OMRI Listed biological fungicide material effective on sclerotia, applied as a spray drench or chemigation. It consists of the natural soil fungus, Coniothyrium minitans. It reportedly attacks the sclerotia survival structures of the fungus in the soil. 

The APS Compendium of Onion and Garlic Diseases and Pests offers white rot guidance: avoid introduction, plant removal, long rotations, avoid over-watering during ideal infection periods 60-65 degrees F, and try and identify varieties reported to offer some field tolerance. UC Davis offers similar advice: http://www.ipm.ucdavis.edu/PMG/r584100511.html

Even conventional agriculture has limited effective options for onion white rot. These include fungicides like Folicur 3.6F (tebuconazole) or pre-plant fumigation with Vapam HL (Metam Sodium) or Telone C-35 (Dichloropropene and chloropicrin). Metam Sodium is among the most widely used crop protection chemicals in the US because of its water-soluble ease of use as a fumigant, solving many problems on a wide variety of horticultural crops. These are all unavailable in organic production.

Buyers beware of gimmicky products. While investigators worldwide test onion white rot control alternatives, they show mixed reliability. High cost, inconsistency, and insufficient control prevent wide adoption of alternatives.

What’s in literature for organic production? Spent mushroom compost had no effect on onion white rot. Onion waste compost materials have reduced sclerotia viability and disease incidence as effective as Folicur fungicide in field trials. These likely work by acting as sclerotia germination stimulants.

Commercial products containing vesicular-arbuscular mycorrhiza, Glomus intradices, were shown to provide season long onion white rot disease suppression nearly as effective as Folicur 3.6F fungicide on organic muck soils of Ontario, Canada. I am not sure if the tested product, MIKRO-VAM, is available as an OMRI Listed product.

Other natural or synthetic fungal germination stimulants have been tested as pre-plant treatments. This is because fungal sclerotia are stimulated to germinate by volatile thiols and sulfides released by soil microorganisms from secretions from the roots of Allium spp. Diallyl disulfide (DADS), or mixing macerated onion products and spraying on fields two weeks prior to planting has been tested; similar in concept to stale seedbed technique for weeds. Biological control with Trichoderma spp. has been tested.