Fungal and Bacterial Disease Aspects in Potato Production - The Potato: Botany, Production and Uses - page 182

SPRING SUMMER

Spores dispersed by
wind, rain splash
and movement of
contaminated soil
on farm machinery.

Germinating spores infect
through natural openings
(lenticels) and wounds.

Sporogenous
hyphae break
into spores.

Spores can infect developing
tubers in soil within a wide range
of temperatures from 50 º to 80 ºF.
Scab is greatly suppressed in
soils with a pH below 5.2.

Mycelia grow through
up to three peridermal
cell layers, causing
cell death.
The pathogen then
feeds on them.

The pathogen
secretes a
compound
that promotes
the formation of
a corky layer.

Common scab of
potato

Sporogenous
hyphae develop
cross walls.

Sporogenous hypha

As the potato
peridem breaks,
a scab forms.

The corky layer
pushes the
infected area
outwards.

Vegetative
mycelium

In spring, vegetative
mycelia develop
specialized sporogenous
spiral hyphae.

As the first cork layer is
penetrated, a new one forms
below, repeating the cycle and
resulting in the development of
large scab lesions.

Pathogen overwinters in the soil and on
infected tubers missed at harvest.

WINTER FALL

Fig. 11.6. The disease cycle of the common scab pathogen, Streptomyces scabies .

of control. Although irrigation may provide an
effective means for managing scab on a small
scale, it may not always be the most practical, es-
pecially for soils with low water-holding cap-
acity. Soil amendments including biological
agents, manure, lime, and cover crops have pro-
duced inconsistent results for the control of scab
(Lazarovits, 2001, 2004; Abbasi et al ., 2007;
Conn and Lazarovits, 2007; Lazarovits et al .,
2008). It has been noted that the use of swine,
chicken, and cattle manures has produced an
initial reduction in the incidence of common po-
tato scab. However, in subsequent years, the in-
cidence has increased (Conn and Lazarovits,
2007). Aside from amendments, soil type and
microbial populations play an integral part of
disease presence.
Chemical and other antimicrobial com-
pounds have been used and vary in their degree of
success. Chemical treatments such as 3,5- D or
benzoic and picolinic acids (McIntosh et al ., 1988)
tend to cause plant injury. Antimicrobial com-
pounds have also been tested for their efficacy on
common potato scab. A few have been effective at

lower rates, but none is registered for use in the
USA. The chemical, pentachloronitrobenzene
(PCNB), has been used with some degree of suc-
cess (Webster et al ., 2011). The use of antagonistic
Streptomyces spp. and other biocontrol approaches
have been shown to decrease the amount of patho-
genic S. scabies present in the soil and to reduce
common scab on tubers (Wanner et al ., 2014).
These antagonistic species have an antibiotic effect
on the pathogenic species (Meng et al ., 2012).
Planting cultivars resistant to common po-
tato scab is probably the best and easiest way to
combat the disease. Genetic improvement from
diploid species may be done for a variety of agro-
nomically important traits, including disease re-
sistance. It is known that certain accessions of wild
potato species have resistance to common potato
scab, although this is not universal within dip-
loid species. These species include Solanum com-
mersonii , Solanum chacoense , Solanum caldasii var.
glabrescens , and Solanum jamesii . Although there
is no variety that is immune to common scab,
resistant varieties can be grown to minimize the
rate of infection (Haynes et al ., 2010).