A few of our pea fields looked a bit pale in comparison to the others earlier this season. The crop colour was off, and not just in low lying spots, as one might expect with all the moisture west central Saskatchewan has received. On further investigation, ruling out other reasons like fertility and plant stand, and after checking out the roots in a few different areas, I suspected aphanomyces.
Before getting into how to sample for aphanomyces, let’s cover some background information on this disease and its effect on field peas and lentils.
The causal pathogen is a water-borne oomycete, Aphanomyces euteiches, which can infect a crop at any time of year. It causes severe damage to roots which can cause premature death of the plant, and can stymie overall growth and nitrogen fixation capabilities. Weakened stems can cause lodging, infected plants can also be delayed in maturity. Symptoms include yellowing of the whole plant, yellowing or wilting of lower leaves, and honey-brown roots with very poor lateral root growth. (Below are examples of discoloured, poor root growth)
What makes aphanomyces different from other root rot pathogens is that it is highly specialized to pea and lentil, and the spores can persist in soil for a long time. Provincial pulse experts have recommended a six to eight year rotation away from peas or lentils if aphanomyces is positively identified in a field. The disease can become even more detrimental and harder to diagnose if fusarium root rot is also present.
Aphanomyces may become an issue due to many factors — the most prominent are a shortened rotation, wet field conditions, and cool temperatures early in the season. Heavy textured soils, compaction, and nutrient deficiencies can also cause additional stress and lead to infection.
To be absolutely sure of the presence of aphanomyces, it’s always a good idea to send some samples away for testing. The materials you’ll need to collect samples are:
- The documentation or requisition form from the lab you intend to send samples to (check with your provincial pulse growers association for a list of labs that can do aphanomyces testing). Provide as much information as you can. It’s also a good exercise in reviewing the underlying reasons why aphanomyces conditions exist.
- Sterile, labeled sample bags for infected soil and plants. It’s helpful to label these before you go out to the field(s). Large and medium sandwich/freezer bags work well for this.
- A shovel and a trowel (Yup, you need both)
- A disinfectant, if you intend on sampling more than one area or more than one field. These are a soil microbial sample after all and the better your sample, the better your result. I recommend a 50:50 water to bleach mixture, soak for 20 minutes, then rinse with water. Or you can use Spray Nine. Remember, you’re trying to limit field to field spread of other pathogens (like clubroot, if you’ve got it, too).
- A cooler with some ice packs. It may seem counterintuitive to keep plant and soil samples cold, but it’s important for accurate testing
- Your phone. Take some pictures of what you’re seeing and you can use your map app to mark where you took the sample from.
Now you’re ready to head to the field. Walk in and find a patch of field where plants have yellowed, or is near a slough. Dig up plants in the area, shake soil off of the roots and bag the plant sample. It’s beneficial to put wet paper towel around the roots. Put the plant sample in a cooler.
Next, dig a hole to take your soil sample. Aphanomyces is sensitive to light, so a more representative sample is collected from four to nine inches below the soil surface. Collect about two cups of soil into your sterile bag using your trowel. Also place in the cooler.
If you can’t get the samples to the lab quickly either by dropping them or in the mail, you can keep them in the fridge until transport. Double check that the form(s) have been filled out and you’re done!