Agriculture Reference
In-Depth Information
crops, then conditions become conducive for
pathogen and disease increase over time and
space.
In developing countries, one major positive
impact of no-till is its advantage, especially in dry
years, to conserve soil moisture, thereby reduc-
ing the risk of crop failure in areas without irriga-
tion. On-farm labor is reduced with no-till,
because land preparation is minimized and labor
is not needed to remove weeds. No-till can lead
to better stand establishment through greater
moisture availability, it reduces turnaround time
to plant a second crop, and it provides a larger
number of benefi cial insects for pest control
(Ekboir et al., 2002). No-till practices led to
higher yields in Ghana with increased food avail-
ability, more time for other activities, and reduced
labor and effort. No-till also expands the markets
for agrochemicals, especially herbicides. However,
no-till technologies may result in threatened
sustainability of the system due to increases in
new weeds, pests, and diseases. Clearly, by com-
bining increased pathogen survival on crop
residue with other factors conducive to disease
increase (e.g., susceptible cultivars and favorable
climate), previous episodic disease problems
can become recurring problems. These factors
can account for the reemergence or intensifi ca-
tion in wheat of powdery mildew, the fungal
leaf spots (septoria leaf blotch, stagonospora
leaf blotch, and tan spot), and Fusarium head
blight.
on both green and senescing host tissue. The cle-
istothecia contain the sexually produced asco-
spores. Because they are the result of recombination,
the ascospores may have new virulence combina-
tions. The cleistothecia remain in crop residue
and are much hardier and longer lived than the
cottony-appearing mycelia and conidia. Asco-
spores require a maturation period and can be
released from cleistothecia in the fall, winter, or
spring to serve as primary inoculum to infect
wheat.
During the growing season conidia produced
on wheat plants are wind-dispersed. Conidia ger-
minate and infect plants under cool, moist condi-
tions. Infection does not require free water on the
plant surfaces, but high relative humidity (near
100%) favors infection. Under optimum condi-
tions, a new crop of conidia are produced every 7
to 10 days.
Identifi cation and symptomology
Powdery mildew on wheat is recognized by small,
white pustules of cottony mycelia (masses of
fungal threads of hyphae that make up the body
of the fungus), conidiophores, and conidia. These
occur on the upper and lower surfaces of the
leaves, as well as on leaf sheaths and heads. As
these patches sporulate and age, they become a
dull tan or gray color. Chlorotic patches may later
surround the mildew colonies. The disease typi-
cally begins in the lower leaves and spreads to the
upper leaves and heads.
Optimum development of powdery mildew
occurs between 15 and 22 ºC air temperatures.
The disease is associated with dense plant growth,
such as high seeding rates and high nitrogen
fertilization, and also with cool, humid weather
conditions. Although favored by high humi-
dity, water on the leaves inhibits conidial germi-
nation; thus long periods of rain will limit
disease development. Heavily diseased leaves
turn yellow and die prematurely. Plants are
most susceptible during periods of rapid
growth, especially from stem elongation through
heading.
POWDERY MILDEW
Taxonomy and life history
The causal fungus of powdery mildew is the
heterothallic ascomycete
Blumeria graminis
(DC)
E.O. Speer f. sp.
tritici
(syn.
Erysiphye graminis
DC ex Merat f. sp.
tritici
E. Marchal). Because it
is an obligate parasite, the powdery mildew fungus
is not typically considered a residue-borne patho-
gen. However, following successive cycles of
asexual, conidial infections on green tissue, the
fungus produces dark, round-shaped cleistothecia
