Reader Comments on the paper
Epidemiology of Karnal Bunt of
Wheat
Blair J. Goates, USDA-ARS,
Aberdeen, Idaho
"...the precise details of many critical steps of disease development are still poorly understood and often are a matter of opinion or conjecture."
-From the paper.
THE FULL TEXT of Epidemiology of Karnal bunt of Wheat
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Gerald Holmes - 09:09pm Jul 5, 1996 EDT (#1 of 10)
Univ. Calif. Coop. Ext. Imperial Co.
This paper was very informative and easy to read. I've read a lot of orignal papers on KB and this settled many issues for me.
Could you speculate as to the survivability of teliospores in Imperial Valley Calif.? In 1983 max daily temperature for the top 1 inch of tarped soil between mid-June and early Aug was rarely below 140 F and as high as 150 F.
You mention that hyphae originating from secondary sporidia are the infecting agents. Can secondary sporidia also infect kernels?
If teliospore germination is not a limiting factor for disease, what is? Is it a combination of factors or do one or two account for most of it?
Do you think KB was
recently introduced into the U.S. (i.e., within the last 15
years) or has it always been around, but just gone undetected?
Gerald Holmes - 07:49pm Jul 6, 1996 EDT (#2 of 10)
Univ. Calif. Coop. Ext. Imperial Co.
Oops! forgot to post this
important question. There seems to be no consensus on the role of
windborne inoculum. Is there good evidence that teliospores are
windblown into the southern U.S. from northern Mexico? My
impression is that teliospores would be too large and heavy to be
windblown long distances except under unusual circumstances
(e.g., windstorms, burning fields). Can someone elaborate?
David Hole - 09:21pm Jul 6, 1996 EDT (#3 of 10)
For Gerald's question on
possibility of teliospore movement: My understanding is that when
fields are burned in Mexico at the end of the harvest, a column
of hot air can raise viable teliospores over a mile into the air.
When that height is coupled with the frequent strong winds from
the south, it doesn't seem unreasonable that spores could move a
couple of hundred miles. Fields infested with KB are located
within 150 miles of the US border. If this is the case, we can
hardly expect to eradicate the disease unless we leave a wheat
free zone around the border. In fact I've overheard some
speculation that APHIS actions in AZ have been calculated to make
it so painful to farmers that none of them will ever want to grow
wheat again :->.
blair goates - 06:31pm Jul 9, 1996 EDT (#4 of 10)
In response to Gerald Holmes. With regard to the survivability of teliospores in the Imperial Valley. I can only say that in general, dry conditions promote longer survival times. The data we have is quite limited and I hate to speculate. Your second question seems to regard solarization techniques. Previous work in India has shown solarization beneath clear plastic to be highly effective (although not quite 100%) after 45 days at the tested depths of up to 20cm. Their data does not give specific temperatures.
Research invloved with the histology of infection has demonstrated that hyphae from secondary sporidia do not directly penetrate the epidermis of kernels.
My car pool is leaving
now, so I will try to get back to answer your other questions
later.
blair goates - 06:29pm Jul 10, 1996 EDT (#5 of 10)
In response to questions from Gerald Holmes. I mentioned teliospore germination as not likely to be a limiting factor in development of KB, partly to contrast KB other wheat bunts. In particular, dwarf bunt does not occur in areas that don't have the specific requirements of long, stable, cold temperature, and moist conditions that are required for teliospore germination. These conditions are provided by a persistent snow cover. This is the reason dwarf bunt does not occur on spring planted wheat. With common bunt, the timing of teliospore germination with the early stages of seed germination is critical.
KB teliospores likely germinate under a variety of temperatures that occur beneath the crop canopy where moist soil conditions can persist between periods of rain or irrigations. Since weather (specifically high humidity) is highly correlated with disease incidence, it appears likely that high humidity allows for the increase of relatively few products of germination (secondary sporidia) which leads to infection.
Several surveys conducted in the southwestern U.S. during the early to mid 1980's that utilized the very sensitive weed wash assay technique demonstrated that spores were not present in wheat sampled at that time. So, I doubt KB has "always been around". Ironically, KB likely could have been detected by these survey techniques shortly after the time the surveys were discontinued. The unusually wet weather of 1993 was probably a key factor in building the disease to levels where it could have been easily detectable with seed washes in many areas at that time. This build-up of inoculum (teliospores) likely caused what we are finding now.
We know viable spores are
present at very high elevation (at least 10,000ft) above the
fields in Mexico while the stubble is burned. Some of these
spores have almost certainly landed in the U.S. however, we don't
know for certain if they were in sufficient concentration to
cause disease. Studies based on the terminal velocity of spores
have indicated spores at 10,000 ft can travel several hundred
miles even in relatively mild wind currents. Given the chance of
large storms and the nature of atmospheric thermodynamics, spores
could easily be transported much further. Fischer and Holton
(Biology and Control of the Smut Fungi, 1957) give some examples
of wind dissemination that have always impressed me. Bunt spores,
at a rate of several hundred per square inch, were found on
leaves of cottonwood trees where the nearest wheat field was 100
miles away and separated by a range of mountains 4-5,000 feet
high. In addition, Tilletia spores have been found at 5,000ft
over the Arctic. Wind dissemination seems to be a plausible
explanation for disease spread.
Gerald Holmes - 12:11pm Jul 11, 1996 EDT (#6 of 10)
Univ. Calif. Coop. Ext. Imperial Co.
I appreciate the responses to my questions.
To Blair Goates: What is
the role of primary sporidia in the infection process? Life cycle
diagrams show primary sporidia causing infection, but it appears
that you put most of that on secondary sporidia.
Kim C. Shantz - 10:47am Jul 12, 1996 EDT (#7 of 10)
Blair Goates - Your observation that KB surveys were conducted by APHIS in the mid 1980's brings several questions.
1. If Karnal Bunt was known to exist in Mexico, and if Arizona was considered the area most likely to become infected, why were the surveys discontinued?
2. With KB detections moving north in Mexico, (Hermosillo in 1992 and Caborca in 1993) and if KB was considered such a threat to American Agriculture, why were the surveys not started again?
3. With the highly
favorable weather conditions for KB development occurring in
Arizona in 1991, 1992 and 1993, why were the surveys not started
again?
blair goates - 05:44pm Jul 12, 1996 EDT (#8 of 10)
In response to the July 11 comment by Gerald Holmes. Some written or drawn descriptions of the KB life cycle do include hyphae from primary sporidia as infecting agents. This comes from the idea that primary sporidia can be moved from germinating teliospores up to spikes from the soil surface by the action of wind or splashing rain drops. I suppose this is possible under the right circumstances but, to my knowledge, none of this is based on any real observations in the field. Spore trapping studies conducted in the field have demonstrated that allantoid (sausage shaped) secondary sporidia, but not primary sporidia (filiform), are present in the air in the vicinity of spikes. Especially very late at night to very early in the morning when the humidity is usually highest. So, I, and I believe most people, think these are the primary source of hyphae for infection. However, I know no scanning electron microscopy studies that show secondary sporidia on field collected spikes. This goes back to my article where I talk about "matter of opinion or conjecture". I have actually watched secondary sporidia "take off" from agar cultures with a microscope. Great fun! They seem to be influenced more by static electrical forces than by gravity.
It would be interesting
to know how many sporidia are on spikes in the field under
conducive conditions and what level of infection can be induced
with different numbers.
Berlin Nelson - 10:48am Jul 28, 1996 EDT (#10 of 10)
A question. Is there some
good information from India/Nepal region on the survival of T.
indica and disease development in mountainous areas where the
soil freezes? Although we think of T. indica as a sub-tropical
smut, I was told there are reports of the fungus in cold areas of
the Himalaya Mts. Are there pathologists from that area who could
provide information? Those of us from northern states would like
some information on the biology of the fungus in cold areas. We
should not assume this fungus is unable to survive or cause
disease in northern wheat growing regions! In the northern Great
Plains the weather in recent years has been favorable for head
infections. My E-Mail is Bernelso@plains.nodak.edu
© Copyright 1996 by the American Phytopathological Society