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Managing crop residue to reduce disease

basal leaves of a harvested cabbage plant with some yellowing from disease
Wrapper leaves left behind after cabbage harvest have active black rot lesions.
Disease could easily spread to the nearby fall cabbage crop.
Photo: M. Grabowski, UMN Extension 
By September, many crops have accumulated some level of leaf spot, fruit rot or other disease problems. Even if these diseases are not severe enough to reduce yields this year, many plant pathogens are able to survive in crop residue from one season to the next, resulting in disease problems in following years. Several basic cultural control practices can be used to reduce the risk of disease spread to other crops in this growing season and the next.

First diagnose any unknown plant diseases in the field. A few plant pathogens, like white mold or Tobacco Mosaic Virus are able to survive exceptionally long periods of time in soil and crop residue. It is important to identify these problematic pathogens early, remove infected plants promptly, and follow sanitation procedures specific to that disease to reduce the ability of the pathogen to survive on site.

While harvest is ongoing, remove infected plants or plant parts to prevent spread to neighboring healthy plants. Many common leaf spot and fruit rot diseases are caused by fungi or bacteria that spread from plant to plant on splashing water or wind. Removing infected plants or plant parts and burying them in a compost pile can reduce the spread of the pathogen through the crop.

For example, remove rotten tomatoes from the plant to prevent spread to immature tomatoes on the vine. Infected fruit should never be harvested with marketable fruit. Rather after harvest of marketable fruit is complete, infected fruit should be collected in a bucket designated for compost, and buried in a compost pile.

rotten watermelon fruit with orange watery fungal spores
Orange colored spores of anthracnose on watermelon can spread on splashing water to nearby fruit
M. Grabowski, UMN Extension 

After harvest is complete for that crop, till plants into the soil as soon as possible. This is especially important if there are crops in the same family in neighboring fields. Many plant pathogens spread on splashing rain or wind to infect nearby plants. Burying infected plant debris below ground reduces spread of the pathogen to other crops and allows the naturally occurring composting soil microorganisms to begin to break down the infected plant debris. This needs to be done in combination with rotation to crops that are not susceptible to the disease (typically a different plant family) for 3 to 4 years. Rotation allows time for pathogens in the infected crop residue to die off.

lower surface of a basil leaf covered with gray spores
Basil leaf infected with basil downy mildew producing abundant spores.
Infected plants should be removed and buried to prevent spread of airborne spores.
M. Grabowski, UMN Extension 

What about high tunnels? Although crop debris can be tilled into the soil after harvest in a high tunnel, many growers find it difficult to rotate away from economically important crops like tomatoes for a full 3 to 4 years. In addition, high tunnel soils are often dry without irrigation which can slow the decomposition of plant material. If rotation is not possible or desirable, it is better to completely remove as much of the infected crop as possible and place it in a compost pile.

Combined with disinfecting stakes, tools, and the structure itself with a commercial disinfectant, this can reduce the population of the pathogen in the tunnel. High tunnel growers should work to diversify their production to allow for a full 3 to 4 year rotation away from susceptible crops as pathogen populations can build up in soil over time even with removal of plant debris.

Author: Michelle Grabowski, UMN Extension Educator
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