Agriculture Reference
In-Depth Information
neighboring leaves and plants. Early blight and
brown leaf spot are largely diseases of older
plant tissues and are more prevalent on senes-
cing tissues on plants that have been subjected
to stresses induced by injury, poor nutrition, insect
damage, or other types of stress. The rate of dis-
ease spread increases after flowering and can be
quite rapid later in the season during the bulking
period and during periods of plant stress.
On potato tubers, germinated spores pene-
trate the tuber epidermis through lenticels and
mechanical injuries to the skin. Tubers often be-
come contaminated with
A.
solani
and
A. alter-
nata
spores during harvest. These spores may
have accumulated on the soil surface, or may
have been dislodged from desiccated vines dur-
ing harvest. Infection is most common on imma-
ture tubers and those of white- and red-skinned
cultivars, since they are highly susceptible to
abrasion and skinning during harvest. Course-
textured soil and wet harvest conditions also
favor infection. In storage, individual lesions
may continue to develop but secondary spread does
not occur. Infected tubers may shrivel through
excessive water loss, depending on storage con-
ditions and disease severity (Stevenson
et al
., 2007).
Effective management of these diseases re-
quires implementation of an integrated disease
management approach (MacDonald
et al
., 2007;
Horsfield
et al
., 2010). These diseases are con-
trolled primarily through the use of cultural
practices, resistant cultivars, and foliar fungi-
cides (Pasche
et al
., 2004; Pasche and Gud-
mestad, 2008). Cultural practices, such as crop
rotation, removal and burning of infected plant
debris, and eradication of weed hosts, helps
reduce the inoculum level for subsequent plant-
ings (Rotem, 1994). Since
A.
solani
and
A. alter-
nata
persist in plant debris in the field from one
growing season to the next, rotation with non-
host crops (e.g. small grains, maize or soybean)
may reduce the amount of initial inoculum
available for disease initiation. Other cultural con-
trol measures may include the following (Steven-
son
et al
., 2007; Wharton and Kirk, 2007):
To minimize tuber infection after harvest, tubers
should be stored under conditions that promote
rapid suberization, as
A.
solani
is unable to infect
through intact periderm.
Cultivars with moderate to good levels of
field resistance to both diseases are available.
However, no immunity to early blight or brown
leaf spot has been found in commercial potato
cultivars or in their wild parents. Highly suscep-
tible cultivars such as Red Norland, Norchip,
and Superior should be avoided in locations
where early blight or brown leaf spot is preva-
lent and disease pressure is high. Field resist-
ance to foliage infection is associated with plant
maturity. Thus, late-maturing cultivars are usu-
ally more resistant than early-maturing culti-
vars, and therefore one should avoid planting
early and late cultivars in the same or adjacent
fields.
The most common and effective method
for the control of early blight is through the
application of foliar fungicides (MacDonald
et al
., 2007; Horsfield
et al
., 2010). Protectant
fungicides recommended for late blight control
(e.g. maneb, mancozeb, chlorothalonil, and
triphenyl tin hydroxide) are also effective
against early blight when applied at approxi-
mately
7-
to
10-
day intervals. Currently, there
are a wide variety of fungicides to choose from
for the control of early blight. However, the
strobilurin or quinone outside inhibitor (QoI)
fungicides are often favored because they offer
broad-spectrum protection against a wide
range of fungal and oomycete diseases, have
reduced environmental impact and reduced
toxicity to mammals and bees compared with
conventional protectant fungicides (e.g. chlo-
rothalonil, mancozeb, and mefenoxam) used
to control early blight (Rosenzweig
et al
.,
2008a,b). Other products that have shown ef-
ficacy against early blight include pyrimetha-
mil (FRAC group 9), boscalid (FRAC group 7),
and fluopyram (FRAC group 7). Spraying
should commence at the first sign of disease or
immediately after bloom, or when spray initi-
ation thresholds have been reached (e.g. the
threshold of
200
P-days (disease severity val-
ues for scheduling initial fungicide applica-
tions for early blight), as described by Sands
et al
., 1979). The frequency of subsequent
sprays should be determined according to the
genotype and age-related resistance of the cultivar.
•
Avoid irrigation in cool cloudy weather and
time irrigation to allow plants time to dry
before nightfall.
•
Use certified, disease-free seed.
•
Use tillage practices, such as fall plowing,
that bury plant refuse.
