An epidemiological analysis of the Phytophthora and Alternaria blight pathosystem in the Natal Midlands.
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Date
1980
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Abstract
The history of the development in Natal of a forecasting
service to warn of outbreaks of late blight disease caused
by Phytophthora infestans is presented. The late blight
pathogen and Alternaria solani, the causal organism of early
blight disease, interact on potatoes and tomatoes to form a
blight disease complex. Evidence is presented to show that
it is expedient to manage this blight complex as a whole
rather than to direct control at only one of the components
in ignorance of the consequential enhancement of the potential
of the other.
In a search for an improved blight complex management
strategy, factors concerning the possible existence of an
annual migration of Phytophthora infestans inoculum, first
postulated in the 1960's, along an east-west route across
Natal, are collected and collated. Corroboration of the
existence of the Phytophthora-pathway is given, inasmuch as
it represents a serial outbreak of late blight along a temporal
gradient. The possibility that the pathway is a
manifestation of disease resulting from the erruption of
pre-existing inoculum along an environmental gradient, can
not specifically be excluded. However, the peculiar pattern
of anabatic and katabatic winds along a river-valley network,
superimposed on a continuous cropping pattern and its
concomitant opportunity for blight to be endemic in the
province, supports the postulated Phytophthora-inoculum pathway A fungicide spray trial was conducted in order to investigate
the possibility of us i ng the pathway phenomenon as the framework
for an improved blight control strategy and to explore
the nature and level of the competitive interaction between
Phytophthora infestans and Alternaria solani. This trial
revealed that the interaction between the components of the
blight complex was differentially altered by weather patterns
and fungicide combinations. Treatments in which metalaxyl
(Ridomil) alone was used for the control of late blight, gave
a yield similar to those with propineb (Antracol), which inhibits
A. solani primarily but also hus some negative effect
on P. infestans. The yields from both these treatments
were siguificant ly (p < 0,05) better than the yields recorded
in the unsprayed control plots. A treatment in which
Ridomil and Antracol were combined such that each was applied
according to its recommended concentration, gave yield increases
of 32,3% over the unsprayed control, although the
yield from the Ridomil/Antracol treatment was not significantly
greater (p < 0,05) than the yields recorded where
either Ridomil or Antracol were used.
A computer simulator, named GAUSE, was developed to simUlate
the consequences of the competition between various combinations
of P. infestans and A. solani. Results simulated by
GAUSE corroborated those obtained from the field trial and
support the conclusion that diseases of complex etiology require
more than simplistic, univariate analysis of single
cause-and-effect pathways. The competition quotient CQ is
developed as a new parameter of competitive interactions. It is calculated as the ratio of the amount of disease in
the absence of competition, to the amount of disease when the
causal pathogen is competing with another pathogen in the
same niche. The CQ may be calculated from various standard
epidemiology statistics and it is used to demonstrate that
the competitive displacement phenomenon places constraints on
the interpretation and application of Vanderplank's basic
epidemiology equations.
A new pathosystems management concept namely the pathotope
(pathos = suffering; topos = place0 concept, is introduced,
having developed from the notion that epidemics have spatial
as well as temporal attributes. Accordingly, an area in
which individual farms are at the same level of probability
at risk to disease, delimits the pathotope. The concept
can be described at many integration lsvels and is presented
as an important quantitative unit of comparative epidemiology.
The pathotope concept accomodates such notions as are contained
in the postulated Phytopnthora-pathway and is especially
suited to integration with disease forecasting methods. An
example of the application of the pathotope approach is
presented and a strategy is proposed by which fungicide
spraying is initiated and applied synchronously as determined
by the degree of communal risk to attack and epidemic increase
of disease.
Within a pathotope, several common factors collectively determine
the vulnerability of the group to disease. If a coherent, uniform strategy is to be developed and implemented
by pathotope members, it is necessary that all members have
access to the relevant information and that it be collected
and disseminated conveniently and rapidly. A computer-based
disease monitoring and mapping system which achieves
these objectives is presented.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1980.
Keywords
Potatoes--Diseases and pests., Theses--Plant pathology.