PRO/PL> Potato purple top disease - USA (WA, OR)

[Jose Garcia de la Rosa]

                INCORRECT  DIAGNOSIS OF THE POTATO
 PURPLE TOP 
           
Ing. Josi Garcma de la Rosa
 
jgarciadelarosa at prodigy.net.mx

Before 1967, many plant diseases were erroneously diagnosed as "virus", due
to ignorance of the existence of phytoplasmas that were discovered that year
by Doi et al in Japan. Besides, in the case of the potato purple top was
reported as a micro nutrients deficiency and, occasionally, as a symptom of
Phytophthora infestans or Fusarium.
 
   
Remission of symptoms in potato with purple top, after several
oxytetracyclin applications, 1987, Toluca Valley.
 
Remission of "rayado fino"  symptoms in corn after the antibiotic
application.  


 
A few years later, potato purple top was proven to be caused by phytoplasma
following the steps of the scientific method,  the fulfillment of Koch
Postulates, and the remission of  symptoms when oxytetracyclin was applied.
Psyllids  and  leafhoppers were proven to be insect vectors of this disease.

 
 
Remission of symptoms  in pawpaw, with symptoms of "virus disease" in the
lower leaves. 2002, Cuautla, Morelos. Mixico.
 
In 1989, Academic Press published the book The Mycoplasmas, Volume V, and in
the Appendix there is a list of approximately 800 diseases caused by
phytoplasmas, which had formerly been diagnosed as "virus". 
 
Nevertheless, during the last 10-15 years there have been, repeated
incorrect diagnoses that consider the following factors as causal agents of
some symptoms of the potato purple top: 
 
TOXINS. Without  isolating the toxin, to prove its  existence and without
making toxicity tests, some researchers assert that the nymphs of
Bactericera(Paratrioza)cockerelli transmit a toxin that causes some symptoms
of purple top. The argument continues, that in order to avoid this damage
caused by a  sucking insect, the most effective method is the chemical
control of the vectors. However, it is accepted that spraying insecticides
is very expensive and the control is unsatisfactory.
 
 
Potato psyllids toxins stunt the growth and cause upward curling of potato
leaves. Photo by Bill Callison.
 
FUNGI. Other investigators have  isolate Fusarium sp., Verticillium sp., or
Phytophthora infestans from  tubers of damaged plants and  concluded that
these diseases can be the causal agent of the spotty  appearance of fried
products. These researchers have ignored that it is not possible to deduce
cause and effects, without fulfillment of  Koch Postulates. Are, the micro-
organisms isolated from a damaged tissue  the causal agent of the symptom
present in the field? In many cases the  micro- organism could be
saprophytic. The same error can be committed when diagnosis is  based only
on  PCR, DNA, ELISA, etc., which can detect the presence of a specific
micro- organism, but that does not prove that it is  the causal agent.
 
VIRUS. After 1967, substantial literature was published proving that many
symptoms attributed to "virus" up  until 1967 were in fact  caused by
phytoplasmas. Moreover, as the "virus" cannot be multiplied in a synthetic
culture media, because they need living cells to replicate, this step
prevented the fulfillment of  Koch Postulates. That is why, some scientists
view that the diagnose of phytopathogenic "virus" doesn't have a scientific
base. However, some researchers still consider the "virus" as phytopathogens
agents, when actually they may only be  phytotoxic.
 
XILELLA FASTIDIOSA. Recently,  it has been reported that this bacteria could
cause some symptoms of the potato purple top with no proof pathogenesis.
This bacteria have been diagnosed as  the causal agent of several diseases
in some fruit trees and is considered as a Fastidious Xylem-Limited
Bacteria(XLB),  without injuring the foliage.  Koch Postulates have not been
fulfilled  in any of these diseases, perhaps, because it is not the causal
agent of the field symptoms.  Also, these  bacteria require polysaccharose,
enzymes, proteins and other organic compounds in order to multiply in a
culture media.  Yet in the xylem vessels only  mineral salts and water
circulate, so wouldn't   X. fastidiosa  be only  a saprophyte?

Other evidence that this bacterium  is not phytopathogen is because the
diseases that it supposedly it causes, can not be controlled with
streptomycin sulphate. Streptomycin sulphate  has been used  as an excellent
bactericide on all crops since 1950
 
 
Pierce's disease of grapevine, supposedly caused by Xylella fastidiosa 
 
 
INSECT VECTORS AND PHYTOPLASMAS. In most universities, insect, taxonomy,
biology, population dynamic and  control with insecticides are studied in
great detail. In a very few cases the research is focused to the direct
control of phytoplasmas with oxytetracyclin. That is why,  potato producers
have limited information on the use of antibiotics to control effectively
potato purple top at low cost. This is true in spite of proven effectiveness
during the last 30 years.

The poor results obtain with insecticides, are due to some of the following
factors: 
1. - The leafhoppers and psyllids have a great mobility, and one insect can
infect 100 plants per day.
2. - They can fly up to 30km. in 24 hours, depending on the wind speed.
3. - Once they insert the stylet in the phloem,  the phytoplasma is
transmitted regardless of whether the insect dies afterwards.
4. - When they suck the sap of plants sprayed with oxytetracyclin, the
phytoplasma that could be carried in the salivary glands are destroyed and
it stops  being an infective vector.
5. - Nobody has determined the economic threshold to justify their control
as sucking insects in potatoes.
.
SYMTOMATOLOGY. The diversity of symptoms cause by phytoplasmas, has produced
confusion to the extent that  potato purple top is often considered a
"syndrome or a pathogenic complex". Recently, based on a symptom named Zibra
chip (Zc), but without knowledge of  either the causal agent or the vector,
it is diagnosed as a "new potato disease", which affects mainly the tubers
quality.
 
In my opinion, the symptoms diversity caused by phytoplasmas, is due to the
fact that the symptoms are not produced directly by these micro- organisms,
but indirectly by altering the sap circulation. 
 
My HYPOTHESIS in this respect is as follows:
"In the autumn, when the temperature drops, the callose (polysaccharose) is
formed in the phloem of some trees,   which cut off the sap circulation and
causes the fall of leaves in winter. In spring, when the temperature raises
the callose is dissolved and the sap circulation is restored and begins the
sprout. In the annual crops, something similar happens: when a strange body
(phytoplasma) enters  the phloem, the plants, as a defense mechanism
produces small quantities of polysaccharose, which partially block the sap
circulation, and that impede the normal movement of nutriments, hormones,
enzymes, etc., causing the symptoms which now we know are typical of the
phytoplasma damage.

When oxytetracyclin is properly applied, the phytoplasma is destroyed, the
polysaccharose disappears, the normal sap  circulation is restored resulting
in the remission of symptoms.
 
In other words, the symptoms are the result of the deficient sap
circulation, and they can be quite different according to the plant age,
variety, moment of the infection, weather, insect vector, etc.

Therefore, the remission of symptoms is the most accurate, quick and
inexpensive method to diagnose and control the damage cause by phytoplasmas.
 

BIBLIOGRAPHY
 
Fletcher, J. and A.  Wayadande.  2000. Fastidious vascular-colonizing
bacteria .  The Plant Health Instructor. 

Flores Nava, A. and Garcma de la Rosa, J. 2002. Potato Purple Top. Pfizer, 
Mixico
 
Garcma de la Rosa, J. 2003. The Kingdom Fungi. Pfizer, Mixico. 
 
Munyaneza, J. 2005. Purple Top Disease in Potatoes of Pacific Northwest of
the US. Yakima Agricultural Research Lab, USDA-ARS. USA 
 
Secor, G.  2005. A new purple top like disease causing losses in potato
crops in North America. Department of Plant Pathology, North Dakota State
University, Fargo, ND USA
 
Whitcomb and Tully, J. G.  1989. Editors, The Mycoplasma, Spiroplasma,
Acholeplasma and Mycoplasma of Plants . Academic Press. New York, EUA

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