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

carbon tetrachloride, applied at the rate of 141 ml m' (4 cc, ft')
of treatment chamber (container), is one of the most effective
chemicals used. The total dosage is applied at 24-hr intervals
over a three-day period in an air-tight chamber filled to not
more than one-third to one-half of its total volume. Samples are
stacked at a uniform height of approximately 4 ft (1.2 m) on
6-in. (15-cm) pallets arranged in rows that allow for ample air
circulation provided by fans. Before treatment, samples are
warmed for five days at 24-25'C, and during treatment
temperature is maintained at 25-26'C; higher temperatures
may result in injury. Rindite is placed in pans above the
potatoes, and burlap bags are arranged to dip into the chemical
and act as wicks. Rindite is highly toxic to humans and proper
safety precautions must be taken.
Certified seed is packed and shipped in clean, new bags or in
bulk in clean, disinfested carriers. Individual bags and carriers
containing bulk shipments are tagged, indicating cultivar, crop
year, seria! or certification number, and grower's name and
address. Tags are attached to each container so that neither can
be opened without breaking the seal.
In the United States all certified seed potatoes are graded in
conlormity with standards established by individual states; in
Canada they are established by the federal government. Grades
vary among states but basically are quite similar to the federal
grade of"U.S. No. I Seed Potatoes" established in 1972, which
serves as a relerence point for marketing seed potatoes,
replacing the U.S. No. I table grade. Most certifying agencies in
North America have several grades, with a blue tag most
frequently representing top quality and other colors for grades
with less stringent standards. Size restrictions, which can vary if
specified, usually range from 111'-3 1 in. (3.8-8.3 cm) in diameter.
with a maximum weight of 12 oz (340 g).
In the United States, fees for certification services are borne
by individual growers, whereas in Canada they are absorbed by
the federal government. Fees cover thecost of application, field
inspections based on acreage, v:rus tests, shipping point
inspections for individual bags or bulk shipments, winter tests,
tags and seals.
Following completion of field and harvest inspections,
agencies publish crop directories listing growers and all varietal
acreages that have met certification standards. Directories are
also released by agencies following completion of the southern
winter tests. Both of these publications, which are widely
distributed within the potato industry, serve as an important
tool in locating reliable seed sources,
Seed potatoes in North America are produced primarily in
the northern states along the Canadian border, in areas of high
elevation in certain western states, and in all Canadian
provinces except Newfoundland. At one time as many as 36
states certified seed potatoes. However, the rapid spread of virus
diseases was the primary reason that southern states dropped
out of seed production. The spread of virus is considerably less
apt to occur in northern areas, primarily because of lower
populations of viruliferous insects, such as the green peach
aphid, which spread many of the viruses that attack potato.
From 1968 to 1977, approximately 78;i of the total seed
production in the United States was produced in Maine (25%),
Idaho (22%), North Dakota (17%), and Minnesota (14%). In
1977, 19 states produced atotal of 230,458 acres of certified seed
potatoes, of which these four states accounted for
approximately 77%,. Prince Edward Island and New Brunswick
are the leading seed-producing provinces in Canada, accounting
for approximately 86%*-of the total acres (66,888) passing
certification in 1977. During the past 10-year period, they have
produced approximately 84% of Canada's seed acreage.
Seed Improvement Programs
Seed improvement programs are constantly striving to
upgrade the quality of nuclear (elite) seed stocks, which serve as
the basis for certified seed potato production.
In an attempt to more effectively control several virus diseases
and bacterial ring rot, four states and four Canadian provinces
have established official Foundation (elite) seed farms where
nuclear seed stocks are developed for their respective seed
industries. The practice of planting cut seed in North America
favors rapid spread of several diseases because the causal
organisms are easily transmitted mechanically; such spread is
curtailed by the planting of whole seed.
These official seed farms are located in well-isolated areas
that havea history of low insect populations and are staffed with
personnel having the expertise to perform the technical
procedures required for development of nuclear seed stocks.
Practices involving strict sanitation, application of systemic
insecticides at planting, and rigid spray schedules for control of
insects and foliar diseases are adhered to at all times.
In recent years, considerable time and effort have been
devoted on these farms to developing nuclear seed stocks free
from such latent viruses as X,S,and M. Virus-free programs
have been developed primarily because of potato virus X
(PVX), which causes latent mosaic. PVX, known as the "healthy
potato virus" because of its symptomless characterisitics,
produces visible symptoms only under certain environmental
conditions, making control by roguing extremely difficult.
Partly for this reason, seed potato certification personnel
welcomed the introduction of virus X-free certification
programs; they fully appreciated that the developing virus Xfree
projects of the 1 9 30s in Europe, which led to such official
programs in America after 1945, were big steps forward in seed
potato improvement. In North America, seed specialists are not
in complete agreement as to the desirability of maintaining
totally virus-free seed stocks. some feel that advantages exist in
the cross-protection provided by mild strains of PVX.
Procedures for the development of nuclear seed stocks vary
on these official seed farms, but in general, programs are based
on clonal selections. A clone is a stock of tubers or plants
derived from the same mother plant by vegetative propagation.
Clonal selection implies the increase of stocks from selected
healthy plants of desirable varietal type and their subsequent
multiplication.
In recent years, meristem and shoot tip culture have been used
to obtain virus-free stocks of standard cultivars, old cultivars of
historical interest, and promising new seedlings. This procedure
is based on the fact that cells in the growing tips ofaxillary buds,
as well as in tips of sprouts, may, by exposure of the plant or
tuber to high temperatures, "grow away" from viruses even
though the plant or tuber is systemically infected. By removing
the meristem tips and allowing them to develop on special
media, virus-free plantlets can be obtained. In practice, a high
percentage of success in obtaining virus-free plants has been
achieved when meristem tips have been taken from rooted stem
cuttings or sprouting tubers that have beenexposed to 35-38 0 C
for 4-6 weeks before bud excision.
Plantlets produced in vitro by meristem tissue culture are not
necessarily pathogen-free and so should be thoroughly screened
for freedom from bacteria, fungi, viruses, and viroids. Some of
the diseases or pathogens for which screening is conducted and
the testing procedures used to detect them are: bacterial ring rot
(eggplant and tomato as indicator plants and the broth test for
plantlets produced in vitro); potato virus X (Gomphrena
globosa as indicator plant and serology); potato virus S
(serology): potato virus A (plant indicator'A6') and potato virus
Y (serology, indicator plants'A6,'and Solanumdemissum P.1.
230579); leafroll (aphid transmission to indicator plant Phisalis
floridana); and spindle tuber (polyacrylamide gel electrophoresis).
In the latter test, the pathogen (viroid) is aribonucleic
acid that differs significantly from the nucleic acids that occur in
healthy plants. Separation by electrophoresis and subsequent
staining permits reliable detection of the viroid. including
strains that cause no visible symptoms.
Stem cutting, which was developed as ameans of eliminating
bacterial and fungal pathogens normally carried over by tuber
propagation, isarelatively new procedure that has proven to be
a valuable tool in seed improvement. Plants from selected tubers
are grown in the greenhouse and then topped when they are
105