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