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

invaders through the IFusarium lesions and rapidly rot the reinainder
of the tuber. Suspended bacteria and juices exuded
from the soft rot endanger surrounding tubers.
Whole-tuber seed becomes infected through wounds during
storage or preparation for planting. The cut surfaces of large
seed tubers are major infection courts. In stored seed tubers,
brown to black flecks appear on the cut surface in about one
week and depressions or pits form in two weeks (Fig. 65A and B).
Mycelia often grow on the depressed surfaces, and undet humid
conditions, depressions may become slimy and black from
bacterial growth. Soft rotting bacteria may also invade through
the IFttsarium lesions and accelerate decay. With numerous
cut-surfacc infections, lesions coalesce: the seed piece rots from
the surface inward: and buds (eyes) are destroved as decay
progresses.
In the field, the shriveling of infected seed tubeis and pitting
of infected pieces may not he evident. The surface over the
lesions is brown, and the underlying necrotic tissues have fewer
cavities. Necrotic tissue may attract soil insects and larvae such
as the seed-corn maggot, which is avector of Erwiniaspecies. In
wet soils, these species often enter as secondary pathogens.
Fusarin spp. alone or in conjunction with Erwinia spp.
partially or completely destroy the seed piece, resulting in
extreme variability in plant size and manl, missing plants (Fig.
651) and E). Often single ;prots emerge: these are small, grow
slowly, produce few marketable tubers, and have a high
incidence of blackleg.
Causal Organisms
1usariun solani (Mart.) App. & Wr. emend Snyd. & Hans.
'Coeruleum and 1.ro'seum (l.k.) Snyd. & Hans. 'Sambucinum'
are most frequently implicated in seed piece decay. In sonic
regions one species is dominant over the other, nut both are
often associated with a seed stock. I. solani is most frequently
encountered and is the more aggressive pathogen. Both grow
and are maintained ott potato-dextrose agar, and the acidified
medium facilitates their isolation when bacteria are present. F
roseu'um grows more rapidly, forming athin, white mycelial mat
and abundant pink to salmon spores. The slower growing .
soani forms a denser, white mycelial mat that exhibits a purple
pigment with age (Plate 48). F.slthnisporulates in culture more
sparsely than does I-'. rosetun (Fig. 66). The optimum temperature
for growth in culture is 20-25°C and for infection,
10-20" .
1:.roseum 'Avenaceum' also causes adry rot of potatoes, but
less frequently than the other species do.
Histopathology
Fusariumn spp. cannot infect intact tuber periderni or
lenticels. Cuts and periderm-breaking wounds incident to harvesting,
storage, grading, and transport are the major infection
courts. Wounds from insect and rodent feeding and frost are
sometimes infected. The Fusaria can also invade surface lesions
of powdery scab, late blight, mop top virus, and possibly other
di:,eases.
Hlyphae are at first intercellular becoming intracellular in
dead cells. In spreading lesions, hyphae may be sparse in intercellular
spaces, with host cells :',owing little reaction to the
fungus. Toward the center of the lesion, less
intercelluar
starch
spaces
is
by
present,
suberin audnt deposited abe inhostcellwalsaandnt
nfetoh
and the myceliuni. usually abundant, may be confined to the
intercellular spaces. In susceptible tissue, starch hydrolysis and
suberin deposition are lacking. Small lesions restricted near the
site of infection may be underlaid by a continuous layer of
wound meristem cells with suberin deposition. With other
isolates, hyphae kill and penetrate cells within two cells of
normal-appearing tissue. Details of the reaction depend on the
pathogen, the resistance of the tuber, and the part of the lesion
examined.
Disease Cycle
Fusariumspp. can survive for several years in field soil, but
the primary inoculum is generally borne on seed tuber surfaces.
Surfaceborne propagules contaminate containers and equipment
used in handling or storing potatoes and enter wounds
incident to handling seed tubers. Infected seed tubersand pieces
decayand infest the soil that adheres to the surfaces of harvested
tubers.
Epidemiology
Tubers of potato cultivars differ in susceptibility to F. solani
and F roseum, but none tested was immune to either pathogen.
Certain cultivars are tolerant to both.
Tubers are tolerant to infection when harvested.
Susceptibility increases during storage and ,:aches its maximumin
early spring about planting time.
Wound healing can reduce infection. Deposition ofsuberin in
the cell walls does not prevent infection, but wound periderm
does. Wound periderm forms in three to four days at approximately
21°C with adequate aeration and humidity but more
slowly at lower temperatures. At 15'C, near optimum for
infection, a period of approximately eight days is required to
form periderm; wound healing is not effective at this cr lower
temperatures.
Dry rot develops most rapidlyin high relative humidityand at
15-20' C. Relative humidities about 70% do not alter rot development,
but lower iiumidities retard infection and disease
development. Disease development continues at the coldest
temperatures safe for potatoes.
If the soil temperature and moisture are suitable for rapid
sprout growth and emergence, seed tuber or piece decay after
planting maybeoflittle consequence. Conditioning seed tubers
from cold storage at 20-25' C for one week before cutting pieces
reduces decay and accelerates sprout growth. Holding contaminated
cut seed several days or weeks before planting or planting
in soils too cold or dry for prompt sprout emergence and plant
growth will accentuate losses. Excessively wet soils after
planting increase secondary infection by Erwinia spp.
Other Hosts
F. solani isolates from Colocasia corms can infect potato
tubers. Generally, tuber-rotting Fusaria do not infect other
plants or plant organs.
Resistance
Differences in resistance exist among potato cultivars. Relative
ranking of resistance isinfluenced by the Fusarium sp. used
for inoculation. Seed lots infected with potato virus X are
relatively more resistant than those free from the virus when
harvested within three weeks of top kill.
Control
1ontrm
I) For storage and seed purposes, harvest tubers from dead
vine
2) Use all precautions with machinery and equipment to
171V' I,
. __
V..
"
,,00 ,
.)'. .
-
Fig. 66. Fusarium solani from culture with many macroconidia
and a single microconidium in center. (x950) (Courtesy P. E.
Nelson)
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