Disease Innoculum, the Gift That Keeps on Giving

From ONvegetables in The Grower, December 2013

Some years the weather patterns just work against us.  Cool temperatures, intermittent rains and prolonged, heavy dews spell paradise for many crop diseases. Under these conditions, many vegetable crops are impacted by diseases such as mildew, scab, fusarium, anthracnose, phytophthora, etc…, despite a well-managed crop and the timely use of fungicides.

What type of impact do last year’s diseases have on next year’s crop?  In part, it will depend on the weather we get next year.  However, rotation is also an important factor.

I came across one source that suggests for every ton of fusarium-infected pumpkins left in the field, 200 lbs of innoculum remain in the soil for the following year! 

For most soil-borne diseases, a crop rotation of 3 to 4 years is recommended.  It is important to stay away from all related crops during the rotation.  In addition, there are other key crops that should not be grown in the same rotation. 

For example, cucumbers and peppers are both susceptible to phytophthora.  Once established in a field, this disease is extremely difficult to manage.  In 2013 we saw a drastic increase in the reports of phytophtora.  This disease has had a tremendous impact on cucurbit crop production in many mid-western states. At this time, there are no fungicide programs that will effectively control this disease.  A long rotation away from both vine crops and peppers will help prevent the development of phytophthora problems in Ontario.

Here are some tried-and-true ways to avoid crop diseases:
  • know your pathogen.  When facing harvest losses, take the time to have the problem properly identified
  • where available, select resistant varieties
  • use treated seed, especially for the common root diseases such as pythium and rhizoctonia. Not all seed treatments protect against the full range of soil-borne diseases.  This is where it is important to know your pathogen when selecting a product.
  • reduce wind damage with windbreaks or wind strips.  Diseases often enter the plant through existing wounds.
  • avoid compaction, plants with poor root growth and low vigour are more susceptible to infection.
  • facilitate drainage: standing water is a plants worst nightmare
  • maintain optimum fertility levels to ensure vigorous, healthy plants.  And finally,
  • aim for thorough fungicide coverage: high water volumes and medium-sized droplets allow the spray to penetrate deeper into the crop canopy.
yellow and wilted zucchini plants
spores from these phytophthora infected plants will persist in the field for 5-10 years.

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