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Buying the best seed for a disease free crop

There are many things to consider when choosing what seed varieties to purchase and where to buy seed from. Which varieties grow well in the environmental conditions on your farm? Which varieties will sell best in your market and which will draw in new customers? It is equally important to consider how the seed you purchase will affect the health of your crop. That great looking variety of squash is not going to sell well if the fruit are covered in soft, rotten spots.

Choice of seed can affect future disease problems on your farm in several different ways.

Plant Disease Resistant Varieties

Each seed company uses its own code to indicate
which diseases a variety is resistant to. Look for
a key to find resistance to a specific disease.
M. Grabowski, UMN Extension 
Choosing disease resistant varieties can stop disease before it starts, improving your yield and reducing the time and money spent on managing disease problems. Due to their specific genetic makeup, resistant varieties have the ability to defend itself against plant pathogens.

Resistance is specific to one pathogen or disease. A cucumber variety that is completely resistant to powdery mildew can be severely infected with anthracnose leaf spot. As a result it is important to know which disease problems result in yield loss on your farm in a normal year. 

During winter months, you can use the UMN online diagnostic tool What’s wrong with my plant? to compare images of common insect and disease problems with issues you observed in your crop last year. Ideally disease problems are identified during the growing season by sending a plant sample to the UMN Plant Disease Clinic for diagnosis.

Look for resistant varieties in seed catalogs or at the Cornell University VegMD site. Resistant varieties are available for many common vegetable diseases.

Resistant Varieties may be:

  • Completely/fully resistant - plants do not develop any noticeable symptoms of disease. In some cases, the pathogen may still infect the disease resistant variety and reproduce in the plant without causing disease (i.e. Vertillium and Fusarium wilt resistant tomatoes). For this reason, planting resistant varieties should not be considered a rotation away from susceptible crops. 
  • Partially or moderately resistant (also marketed as tolerant) – disease develops in the crop but it develops slowly or is restricted. Yield loss from disease is reduced due to partial or moderate resistance. (i.e. powdery mildew tolerant pumpkins, slow rusting asparagus) 

Purchase Clean Seed 

Plant pathogenic viruses, bacteria, fungi, and nematodes can be carried on or within seed. New plant pathogens can be brought onto the farm on infected seed. In some cases, these pathogens result in disease problems for years to come.

Disease arising from infected seed often begins very early in the growing season. This provides ample time for the disease to spread and can result in severe damage to the crop.

Ask about Seed Treatment

There are several different types of seed treatments that can be done to eliminate pathogens from seed. This may include treatment with hot water, chemicals like trisodim phosphate, certain acids, or bleach or other treatments. Most seed companies do not include information about seed treatment in their catalogs. A few companies provide seed treatment information on their websites.

I have found that the best way to find out about how a company treats their seed to eliminate pathogens is to speak with a customer service representative. It is best to be specific about what diseases concern you as there is no one treatment that eliminates all pathogens.

Last year I contacted a number of seed companies by phone, email, or using an online customer chat box to ask about how the company treated seeds of cole crops to eliminate the bacteria that causes black rot. The responses were incredibly diverse. Some companies did no treatment at all, others would treat seed upon request only, while still others treated all seed automatically.

If a disease is a concern it is worth taking some time in the winter months to determine if the pathogen can move on seed and if so what companies are doing to prevent that possibility.

Tested or Certified Seed

This catalog advertises that seed potatoes are tested for virus and certified following the state of Maine's protocols
M. Grabowski, UMN Extension 

There are several tests that can be conducted to determine if a pathogen is present in seed. Some companies will test seed lots for the presence of known seed borne pathogens. Seed that is shipped internationally may require testing in order to verify that it is free of pathogens that do not occur in the country of import.

It is important to understand that these tests are often destructive. That means a sample of the seed is taken from the seed lot and tested. The seeds used for the test are destroyed by the process. If negative, the rest of the seed lot has ‘tested’ to be free of a specific pathogen. Because each individual seed cannot be tested there is always some risk that the pathogen is present in a seed lot that has tested negative for a pathogen but it is likely to be at a very low level.

Seed may be marketed as 'certified' disease free. This typically means that an independent agency has tested the seed using a set protocol to determine if plant pathogens are present. This is common in seed potatoes. Be aware that different certifying agents may have different protocols and may test for different pathogens.

Talk to a customer service representative or visit the certifying agencies website to find out what pathogens are tested for and how the seed is tested.

Information about seed testing may be available on a company’s website but more often you will need to talk with a customer service representative. Do not expect a guarantee of disease free seed, however, as only a sample of the seed can be tested.

Fungicide Treated Seed

Fungicide treated soybean seed is dyed pink.
M. Grabowski, UMN Extension 

Seed can be treated with several different fungicides prior to sale. Fungicide treated seed will have a distinct color (bright pink or blue) to indicate that it is not safe to eat. The fungicides used in seed treatments are designed to protect the young seedling from damping off, a fungal disease that causes seed rot and kills seedlings. These fungicides will not protect the mature plant once it has grown out of the seedling stage.

Growers using fungicide treated seeds should read the seed packet and accompanying information carefully and follow any instructions provided. General guidelines for the safe handling of pesticide treated seed can be found at the MN Crop News.

Michelle Grabowski

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