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High Mowing Organic Seeds
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The Seed Bin - May 2011


Seedborne Disease and its Control - Jodi Lew-Smith, Ph.D. Director of Research and Production

Some plant diseases are spread through the actual seeds themselves. When customers buy our seeds, we do our best to ensure that these seeds are disease-free. Non-organic companies can spray their seeds with chemical fungicides to kill off the diseases, but organic companies such as ours need to utilize different tactics. The following article by Dr. Jodi Lew-Smith, our Director of Research and Production at High Mowing Organic Seeds, details the different types of seedborne diseases and how we treat them.

Types of Seedborne Disease Pathogens

Seedborne disease refers to the particular plant diseases that are transmitted by seed. In some cases the transmission on seed is insignificant compared to the population of disease organisms that exist in soil or on weed species. In other cases, the transmission on seed is the primary means by which a disease spreads. While we are cautious about any type of disease on seed, it is this latter set of diseases that we must be most vigilant in controlling. For the purposes of this article, we will call them seed-specific diseases.

Diseases of plants are caused primarily by three types of pathogens: bacteria, fungi, and viruses. Despite that fungi comprise the largest group of pathogens, the bulk of seed-specific diseases are caused by bacteria or viruses. This is due to the fact that bacteria and viruses are more adept at entering and then traveling through the veins of the plant, a phenomenon known as ‘systemic infection,’ and from the vascular system may make their way into the developing embryos of seeds.

Fungi, in contrast, tend to be restricted to the outer layers of the plant, where they initiate infection by means of air-borne spores and then proceed to spread by attacking nearby cells of the outer layers. Fungi are much less likely to enter the vascular system of the plant, and thus infect seed mostly when they either ‘crawl’ all the way to seed on the outside of the plant, or else send out spores that land on the seed. In either case, the fungal spores are on the outside of the seed, in the layers of the seed coat. Spores on the seed coat are more prone to either dry up and die, or else to get sloughed off with the seed coat during seed germination, thereby failing to cause disease on the next generation of plants.

Treatment Methods for Seedborne Disease

Disease pathogens restricted to the seed coat are treatable by external application of anti-microbial agents such as bleach, acid, trisodium phosphate, or other commercial products. Rarely do these treatments effect 100% sterilization, but they can greatly reduce levels of pathogens. These types of treatments are typically used for the class of non-seed-specific diseases in which seedborne transmission is minor compared to the levels of inoculum already present in soil due to crop debris. An example of such a disease is cucurbit scab (Cladosporium cucumerinum), a fungal disease which tends to flare up in wet years on fields that have grown cucurbits repeatedly.

Seed-specific disease pathogens that reside inside the seed, which are typically bacteria or viruses, cannot be eliminated by surface sterilization. Because they’re often inside the embryo itself, these pathogens are almost certain to divide and spread to cause infection when that seed germinates and grows. They cannot be eradicated by external application of chemicals, however, they are susceptible to the one agent that can penetrate the interior of the seed, which is heat. The number one method for sterilizing seed is to treat it with either wet or dry heat, which penetrates to the core of seed. Heat kills the majority of bacterial and fungal pathogens, and bacterial pathogens are particularly sensitive to heat. Wet heat, in the form of hot water, is more effective than dry heat, and thus the most common method for treatment of seed disease is hot water of 122ºF (50ºC) for 20-25 minutes. We have found in our own lab, though, that temperatures of 118ºF (47ºC) are equally effective for most pathogens and less damaging to the seed.

Hot water is commonly used for treatment of most small seeds, but is less effective and more difficult to use for large seeds. Large seeds tend to be damaged by wetting and re-drying, are more difficult to penetrate fully with heat, and are so bulky as to make it difficult to efficiently wet and dry them. 

Unfortunately, viral pathogens are generally not susceptible to heat, although dry heat has been shown to have some efficacy against certain tomato viruses. Solutions of bleach or trisodium phoshphate are sometimes used to remove surface infections of virus in pepper and tomato seed. In general, though, viral pathogens are quite difficult or impossible to remove from seed, and thus virus-diseased plants in a seed field are almost always pulled up and destroyed immediately.

Specific Seedborne Diseases and their Control

A.    Black Rot

At High Mowing we take seed borne disease very seriously. We believe most seed companies will agree that the number one seedborne disease of concern is black rot of crucifers. Black rot, caused by the bacteria Xanthomonas campestris pv. campestris (Xcc), is highly virulent to all crucifers - the group that includes broccoli, cauliflower, cabbage, Brussels sprouts, kale, collards, turnips, and many different salad greens. Because it is so deadly, it tends to kill its host plant if it infects the plant early. Whole fields are quickly wiped out if black rot takes hold early in the season. If you think about this from the pathogen’s perspective, you’ll realize this gives the pathogen a unique problem: How do I get myself over to a new field of healthy crucifers once I’ve killed all the plants in this field? I can’t live in soil, I need live tissue to survive, so golly, I really shouldn’t have murdered all my hosts. But oh, wait, across the hedge line, in that field over there. I see some plants that don’t look sick yet and they’re flowering. I think I’ll mosey my way over there and get up in those flowers in time to get into the seeds. I know I can live in seeds, and seeds will take me a long way away to where there are certainly more hosts to infect.

OK, enough for taking this so seriously! But the point is that it takes a late infection for black rot to infect seed, giving the plant time to grow to full size, flower, and set seed before black rot infects. This means the most likely scenario for seed infection with black rot is a seed field that looks perfectly healthy most of the season but becomes infected sometime after flower stalks emerge, thereby allowing a small proportion of the total seed crop to be infected.

For this reason, we and other seed companies are exceedingly fastidious about testing for black rot. Larger seed production companies who grow crucifer crops all have their own labs for testing seed using high tech methods for detection of proteins specific to the black rot bacteria. All of our hybrid crucifer seed is tested in this manner, by the companies that produce the seed and are themselves vigilant in preventing black rot.

In the past year a new tool has also become available, a simple-to-use strip test, similar to a pregnancy test, that detects black rot proteins in ground samples of seed. In our High Mowing lab we now use these strip tests for all small lots of crucifer seed that we have grown by our own contract seed growers. The strip test, together with a sophisticated sampling technique that takes seed from all portions of the lot, assures that the seed is free of black rot down to the sensitivity level of the test. Note that no test guarantees 100% absence of disease in every last seed of the lot, as you can only ever test a sample.

We have yet to have a seed lot test positive for black rot, however the one good aspect of black rot is that the bacteria is highly sensitive to heat, and crucifer seed is very easily treated with hot water. This means any lots for which there is any reason whatsoever to suspect disease can be preventatively treated with hot water. Most larger high-value seed companies routinely treat all crucifer lots with hot water.

Tomato Mosaic Virus

Another highly infectious seedborne disease is tomato (or tobacco) mosaic virus, commonly called TMV. Unlike black rot, TMV rarely kills its host plant, making it that much easier for the virus to enter the seed and get passed along. This makes TMV in seed much more common than black rot, and thus that much more of a problem.

Luckily for growers, though, for some varieties the symptoms of low-level TMV infection are nearly undetectable, making it a non-issue for tomato growers. As a seed company, though, we feel it our responsibility to never knowingly sell seed infected with disease. For this reason we use a similar strip test to the one described above for black rot to test all incoming tomato stock seed for our own productions to make sure it is TMV-free before planting for seed crops. If virus symptoms are detected during the growing season, we use the same test to assay for virus in the leaves. If it is detected, we will pull and destroy the plants immediately. Similarly, all of our hybrid tomato seed is watched closely throughout the growing season and then tested either by us or by the companies that produce it for sale, assuring it is free of TMV.  

Other Tested Diseases

The reality of the seed business is that disease testing is expensive and thus reflected in the cost of the seed. The higher-value the seed, the greater the number of tests it receives. For lower-value seed, the primary strategy is prevention. This means:
  • growing seed crops in ideal climates whenever possible,
  • making sure all growers are responsible about walking crops to scout for signs of disease and then pulling up any with confirmed disease,
  • using only disease-free stock seed for planting productions,
  • and performing preventive applications of organically-allowable sprays where necessary.
  • Tomato seed is also routinely fermented during harvest, a process that eliminates nearly all surface pathogens.

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