Compendium of Potato Diseases - (PDF, 101 mb) - USAID

The "yellows" types of disease, characteristic of mycoplasma
infections that have been studied in some detail possess rather
broad host ranges. Some have been reported to infect potato in
the field or have been transmitted to potato experimentally.
Unfortunately, disease symptoms in the potato itself are hardly
of diagnostic value except to distinguish two large groups of
agents, those causing the aster yellows and the witches' broom
types of disease.
Mycoplasma-like organisms (M 1.0). formerly considered
viruses with somewhat unusual characteristics, were first
demonstrated in plants in 1967 by electron microscopy. Actual
proof that MI1.0 cause disease in potato is needed by Koch's
postulates, although the presence of MLO in plant tissue
constitutes the best evidence so far available. More accurate
methods of differentiating between these pathogens are urgently
needed, particularly because potato is a relatively incompatible
host.
None of these diseases is contact-transmissible. Grafting is
often used for experimental transmission. All of the pathogens
rely on leaflioppers for transmission and dispersal, and their
occurrence and distribution is determined by lealfiopper
activity. Variations in symptomatology, host range, or vector
relations of different source materials of each disease suggest
that strains of these pathogens occur.
Symptoms and M I. may be suppressed by antibiotics of the
A7
Mycoplasmas
tetracycline group. and individual plants may be cured by heat
therapy.
Aster Yellows and Stolbur
Aster yellows and its allied diseases occur worldwide. Stolbur
is found in Europe, tomato big bud in Australia, purple top roll
in the Indian peninsula of Asia, and parastolbur and
metastolbur in Europe. In the western hemisphere, aster yellows
has been variously called purple top wilt, yellow top, bunch
top. purp!e dwarf, apical leafroll. haywire, latebreaking virus,
blue stem, and moron.
Symptoms
Upper leaflets roll and develop purple or yellow pigmentation
(Plate 77). Aerial tubers are common, and occasionally some
proliferation of axillary buds occurs (Plate 78). Often only a
single stem in a hill isaffected. Plants are often stunted and may
die prematurely(Fig. 97A and B). Under field conditions, lower
stems frequently develop cortical necrosis, shredding of tissue,
and vascular discoloration.
At harvest, an affected hill usually has some normally mature
and some immature tubers. In stolbur, flaccid (gummy) tubers
Fig. 97. Aster yellows mycoplasma in potato: A, primary symptoms; B,advanced wilting of stolbur in left stem with stem at right unaffected;
C, aster yellows mycoplasma in infected tissue of aster (electron microscope photograph, bar represents 1pm. (A and C, Courtesy N. S.
Wright; B,courtesy V. Valenta)
44;
91