by Tamara Scully
Seeds treated with antimicrobial or antifungal chemicals are often planted to help growers combat seed-borne diseases. If seed-borne diseases do occur, there may be treatment options to treat disease in the field. But once in the field, seed-borne diseases can spread via other routes of infection, and preventing inoculum from infected seeds is key.
For organic growers, where effective in-field treatments may be limited, preventing seed-borne disease from taking root is crucial.
“In organics, we don’t have that kind of toolkit that you have in a conventional system,” Jodi Lew-Smith, of High Mowing Seeds, said in a recent webinar. “Our key strategy is preventing disease whenever we can.”
Diseases of primary concern to organic growers can be either highly seed-borne and mildly virulent, or mildly seed-borne but highly virulent. The worst are highly seed-borne, highly virulent diseases, which “can pretty much wipe out your crop,” Lew-Smith said.
Organic growers who are concerned about possible seed-borne pathogens can germinate some of their seed pre-planting, to assess whether disease is present in the emerging seedlings.
“Look at the seedlings and see if there’s disease that came from the seed,” Lew-Smith said, noting that signs of disease can be noticeable on the cotyledon, stem or roots of the seedlings.
Bacteria, fungi and viruses cause disease in plants, and some of these diseases can be carried on seeds. While bacterial diseases are often the most virulent in the fields, they are readily treatable on seed. Seeds infected with fungal pathogens are not as easily treated. Viruses, although not as destructive in the field as either bacteria or fungi, can’t be readily treated in seeds, or in the crops themselves, in organic systems, according to Lew-Smith.
Bacteria are very heat-sensitive, simple-celled organisms. They can only survive inside a host insect or inside of the plant. If they are present in the seed, heat treatment can readily destroy them. Fungi are hard-walled, and not sensitive to heat, making them difficult to kill with organic seed treatments. Viruses are merely DNA or RNA that make proteins. These different proteins can be detected by strip tests that can be used before planting. If viruses are present in the seed, organic seed treatments are not effective.
If pathogens are already present when plants begin to grow, diseases tend to be severe. Plants infected young are more likely to succumb. Basil downy mildew, caused by a fungus, is an example. The disease was spread rapidly via infected seed, causing severe economic damage to growers. Planting infected seeds or transplants into crop fields can cause long-term problems, as non-seed-borne sources of inoculum — such as the soil or plant debris — will now harbor the pathogen, often for years.
Some pathogens reside on the outside of seeds. Others, such as Peronospora belbahrii, responsible for basil downy mildew, are able to enter the seed, making them difficult to eradicate. Cucumber mosaic virus, lettuce mosaic virus and Botrytis spp. in onions are some disease which actively enter the seed.
Seeds can be treated using hot water, aerated steam, chlorine, or hot and dry air. This physical seed treatment needs to kill the pathogen, but not the seed. Biological, organic or conventional fungicides can also be used to treat seeds. Biological products work by parasitizing the pathogen. Fermentation can promote growth of beneficial bacteria and is often used with tomato seeds. Effective treatment depends on the type of seed and the pathogen.
Virulent seed-borne diseases
The worst of the seed-borne diseases seen in Brassica crops are black leg and black rot, Lew-Smith said. Black rot is a bacterial disease, caused by Xanthomonas campestris. It rapidly spreads in warm, humid weather and results in the “melting” of the plant. It’s signature triangular shape, always seen at the leaf margins, is distinctive.
Black leg, a fungal disease, also spreads in warm and humid conditions, and its spores will survive for years in the field. It is recognized by black spots on the stem or upper roots of the plant. These spots are fungal bodies, and will release a large amount of spores, perpetuating the disease.
Lettuce mosaic virus is “very seed-borne” and highly virulent, Lew-Smith said. “It won’t kill the whole plant, but it will persist year after year after year in seed.”
There is a strip test that will detect this virus. In typical virus fashion, symptoms appear as a mottled look on the plant. Some lettuce varieties can be asymptomatic carriers, so testing seed before sowing is important.
Alternaria leaf blight in carrots is only moderately seed-borne, but can cause large reductions in yields. It is also of concern in beets.
“The damage is mostly by losing so much leaf material that you lose your yields,” Lew-Smith said.
Tomato mosaic virus is highly seed-borne and hard to eradicate from seeds. There is a strip test. Infected fruit is unmarketable. Three other highly seed-borne diseases of concern in tomatoes are bacterial speck, bacterial canker and bacteria spot. These can be eradicated by steam treatment or fermentation of the seeds.
White rot, a fungus, is commonly found mixed in with onion seed, although it does not live in or on the seed itself and is not a true seed-borne disease. The problem arises because the fungus’s sclerotia look like small black dots, and can’t be readily distinguished from the onion seed, which are also small black dots. Sclerotia, which are fungal reproductive bodies, can last more than a decade in soils, so preventing white rot from getting into a field is essential.
Treating seeds prior to planting is only the first step in reducing disease spread. Because seed-borne diseases also have other modes of infection, and can survive in soils and plant debris, field sanitation remains important. Pathogens can often be introduced via water or wind, or insect vectors. Seed saved from fields where the pathogen is present can also carry seed-borne disease and the cycle continues.
A list of seed-borne diseases in vegetables is available at https://tinyurl.com/y9fddp39.