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

(Fig. 34B). Bacterial streaming can also be seen rnicroscopically
in thin sections of infected tissue mounted in water under a cover
slip. This bacterial exudate, combined with wilting and related
symptoms, distinguishes this wilt from fungous wilts,
Underground stems, stolons, and roots of plants with initial
foliage symptoms show few advanced symptoms of infection,
Grayish brown discoloration, usually evident through the tuber
periderm, indicates well-established infection. Tubers from
infected plants may or may not show symptoms: cross sections
usually show distinct, grayish brown vasculardiscoloration that
may extend into the pith or cortex from the xylem tissue,
However, certain strains from Portugal and Kenya produce no
browning of the vascular ring.
When tubers are cut in half and light pressure is applied,
grayish white droplets of bacterial slime ooze out of the vascular
ring (Fig. 34A). The eves. often at the bud orapical end, become
grayish brown, and a sticky exudate may form on them or at the
stolon connection (Plate 15). The bacterial ooze mixes with the
soil, causing soil particles to adhere to the tuber surface. An
infected tuber left in the ground continues to decay; secondary
organisms convert it to a slimy mass surrounded by a thin laver
of cortex and periderm.
Causal Organism
Pseutdomonas solanacearum F3. F. Smith is a nonsporeforming,
noncapsulate, Gram-negative, nitrate-reducing,
ammonia-forming aerobic, rod-shaped bacterium. In liquid
media, the wild-type bacterium is usually nonmotile and does
Fig. 34. Brown rot: A, bacterial exudate from vascular ring of
tuber; B, streaming of bacteria from infected stem in water; C,
cells of Pseudomonas solanacearum (electron micrograph) of
avirulent type with wavy flagellum; D,virulent form (flagellum is
atypical). (A,Courtesy L.W. Nielsen; B,courtesy C.Martin; C and
D, courtesy A. Kelman)
30
not form a polar flagellum. Avirulent variants that develop in
culture are actively motile (Fig. 34C and D).
Starch is not hydrolyzed by this bacterium, and gelatin is
liquefied slowly or not at all. . solanacearuniis sensitive to
desiccation and is inhibited by relatively low concentrations of
salt in broth cultures. Optimal growth of most strains occurs at
30-32' C,although some strains from Colombia grow relatively
well at lower temperatures.
Strains differing in biochemical characteristics and host range
have been described. A strain pathogenic to potatoes is weakly
virulent on tobacco but avirulent on banana; the banana strain
is avirulent on potato; in contrast, tobacco and tomato strains
are usuall \iitnllI potato. Somic strains of P..hnacearinm
from Portugal and ..*;iya do not forl the typical lyrosinase
reaction in cult ure tiedia.
Cultures of . o/tltlcealIII)rlll aintinied io unicratcd liquid
ilcdia rapidly lose kirulcnce and \iahilit and shilt tron the
Iltuidal (11oni6otilC) \\ill-tvp'C to a\ irulnt. highl\ Motile
variants. Colonies of virulent wild-types are irregularly round
and are white with pink centers; colonies of avirulent variants
aic uiiifolumly round, butyrous, and deep red. Colony characteristics
are best observed when cell suspensions are streaked on
plates of peptone casamino acid glucose agarcontaining 2,3,5triphenyltetrazolium
chloride and examined in obliquely
transmitted light after incubation at 32'C for 36-48 hr.
Epidemiology
In tropical and semitropical regions (southeastern Asia,
Central and South America, and parts of Africa and Australia)
the pathogen can be borne by tubers; quarantines exist against
importation of seed potatoes in sonie areas. Infected seed
potatoes are an important factor in the distribution and
increasing severity of the disease in tropical countries such as
Peru, where latent infections can occur in seed grown at high
elevations.
Temperature plays an important role in the geographic
distribution of the organism, which is rare where mean soil
temperaturesare below 15' C. In North America, seed potatoes
are grown in the temperate regions where P. solanaearumdoes
not occur, and tuber transmission is not a problem. tligh temperatures
favor growth of the pathogen in vitro and
development of the disease in the field. Recently, however,/:
solanacearutn was reported in Sweden and at high altitudes in
Costa Rica, Colombia, Peru, and Sri l.anka. Thus, the
bacterium can possibly survive and infect potato crops at
relatively low temperatures.
The disease occurs in soil types ranging from sandy to heavy
clay and over a wide range of soil pH. Disease usually develops
in localized areas often associated with poor drainage. On newly
cleared forest land, bacterial wilt may be severe if a susceptible
crop is planted.
Other Hosts
Important economic hosts of P. solanacearum include
tobacco, tomato, pepper, eggplant, peanut, banana, and a
number of ornamentals and weeds. Although species in over 33
different plant families may be attacked, most susceptible hosts
are in the Solanaceae.
Resistance
At least three dominant and independent genes control
resistance in potato to certain strains of bacterial wilt.
Resistance is relatively sensitive to changes in environment;
increased temperatures and decreased light intensity enhance
susceptibility to wilt.
Immunity or high levels of resistance have not been identified
in clones of S. ttu/erosum. Colombian clones of S. phure/aJuz.
& Buk. with resistance to bacterial wilt have been crossed with
haploid lines of S. tuberosuim. Strains of P. solanaceartan
differing in virulence have complicated breeding for wilt resistance,
but two resistant varieties, Caxamarca and Molinera, have
been released in Peru.