Agriculture Reference
In-Depth Information
Infection of potato tubers by pathogens
that cause dry rot occurs through wounds in-
flicted during harvesting, grading, cutting, and
handling of tubers (Secor and Salas, 2001).
Thirteen species of
Fusarium
have been impli-
cated in fungal dry rots of potatoes worldwide
(Hanson
et al
., 1996; Cullen
et al
., 2005; Peters
et al
., 2008a), of which eight species have been
reported in the northern USA (Hanson
et al
.,
1996). The most important species infecting po-
tatoes are
Fusarium sambucinum
Fuckel; (“syn”
Fusarium sulphureum
Schlechtend: “tel”:
Gibber-
ella pulicaris
),
Fusarium solani
(Mart.) Sacc. var.
coeruleum
(Lib. ex Sacc.) C. Booth (“syn”
Fusar-
ium coeruleum
; “tel”:
Nectria haematocca
), and
Fusarium oxysporum
Schlechtend. Fr. (Hanson
et al
., 1996; Ocamb
et al
., 2007). Other species
reported in the northern USA include
Fusarium
avenaceum
(Fr.) Sacc.,
Fusarium culmorum
(W.G.
Smith),
Fusarium acuminatum
Ellis & Everh.,
Fusarium equiseti
(Corda), and
Fusarium crock-
wellense
L.W. Bugess, P.E. Nelson & Ravenel (
Fu-
sarium cerealis
) (Hanson
et al
., 1996). Most of
these species were also recovered in a study in
the Pacific region of the USA, with
F. sambucinum
being the most prevalent (Ocamb
et al
., 2007).
Recently,
Fusarium graminearum
was reported to
be the prevalent
Fusarium
species causing potato
dry rot in North Dakota (Estrada
et al
., 2010).
Fusarium dry rot on seed potatoes can be
controlled at two phases during the potato
growth cycle
(Fig. 11.3
): (i) control of seed piece
decay prior to planting, which is achieved primar-
ily by seed treatment (Wharton
et al
., 2007b);
and (ii) postharvest control of dry rot on both
stored commercial tubers and seed tubers (Nolte
et al
., 2003). Fludioxonil (Maxim
®
Seed Potato
Protectant; Syngenta Crop Protection, Greens-
boro, North Carolina, USA), a phenylpyrrole, is
among the few fungicides registered for potato
seed treatment against Fusarium dry rot in the
USA and Canada. Recently, fludioxonil-resistant
strains of
Fusarium
spp. were reported in Canada
and the USA, including strains of
F. sambucinum
,
F. oxysporum
, and
F. coeruleum
(Peters
et al
.,
2008b; Gachango
et al
., 2011). Sensitivity to flu-
dioxonil in other
Fusarium
spp. that cause potato
seed piece decay is not known.
Postharvest management of potato dry rot has
been achieved primarily by reducing tuber bruis-
ing, providing conditions for rapid wound heal-
ing (Secor and Johnson, 2008), and applying
thiabendazole (TBZ), a benzimidazole fungicide
(Mertect 340F
®
, Syngenta Crop Protection), as
tubers are placed into storage (Hide
et al
., 1992;
Ocamb
et al
., 2007). Isolates of
F. sambucinum
resistant to TBZ were first found in Europe (Hide
et al
., 1992), and subsequently in the USA (Des-
jardins
et al
., 1993; Desjardins, 1995). Fungi-
cide resistance has reduced the efficacy of TBZ in
controlling
F. sambucinum
, but may be useful
where TBZ-sensitive isolates still occur. To coun-
teract the reduced effectiveness of TBZ and, po-
tentially, of fludioxonil associated with resistant
strains of pathogens, new registrations of efficacious
postharvest fungicides with differing mechanisms
of action (Fungicide Resistance Action Commit-
tee (FRAC) groups) are needed. One fungicide
that has been tested with promising results is
difenoconazole (FRAC Group
3;
Inspire
®
; Seed
Potato Protectant; Syngenta Crop Protection), a
sterol biosynthesis inhibitor (Adaskaveg and
Förster, 2010; Gachango
et al
., 2012a).
Role of soil inoculum in potato disease
The occurrence and development of potato dis-
ease depends on the mutual concurrence of sev-
eral diverse factors affecting either the pathogen
or the plant, but is fully dependent on the sur-
vival and spread of inoculum. The role of the
tuber in some potato diseases has already been
discussed. Long-term survival of pathogens is
also dependent on the survival of inoculum in
soil and on other types of media, such as in stor-
age structural material. Fungal spores such as
conidia of
Helminthosporium solani
(silver scurf)
may survive in dust in storages, but may colon-
ize wood and other materials such as foam insu-
lating sealant (FastFoam
™
) and plywood (Frazier
et al
., 1998). Although storage structural mater-
ials and mechanical devices used in potato pro-
duction are important locations and vehicles for
the dissemination of inoculum, the most com-
mon medium for inoculum survival is the soil
(Carnegie
et al
., 2003; Al-Mughrabi
et al
., 2007;
Gudmestad
et al
., 2007; Johnson and Dung,
2010; Hannukkala, 2011; Fiers
et al
., 2012). In-
oculum of many pathogens survives in the soil
independently of tubers (Fiers
et al
., 2012). Favor-
able conditions for survival, germination, and
infection of many potato pathogens correspond to
