emerging plant disease

1. HARAMAYA UNIVERSITY
SCHOOL OF PLANT SCIENCES
Plant Disease Epidemiology and Management (PLPA 762)
PHD Program, Plant Pathology
An Assignment
of
Articles Review on
VARIATION IN THE RESISTANCE SPECTRA OF ETHIOPIAN WHEAT CULTIVARS TO STEM
RUST (PUCCINIA GRAMINIS F.SP. TRITICI) AND ITS EPIDEMIOLOGICAL IMPLICATIONS
By
Samuel Tegene(PhD 020/08)
and
Dawit Getahun (PhD 020/08)
October, 2016

2. VARIATION IN THE RESISTANCE SPECTRA
OF ETHIOPIAN WHEAT CULTIVARS TO
STEM RUST
(PUCCINIA GRAMINIS F.SP. TRITICI) AND ITS
EPIDEMIOLOGICAL IMPLICATIONS
By
Naod B., Chemeda F. and Ayele B.

3. Typical Symptom of Stem rust(Puccinia graminis f.sp. tritici )

4. Life cycle of stem rust (Puccinia graminis f.sp. tritici )
Puccinia graminis f.sp. tritici is heteroecious, that is, it has a parts
of its life cycle on wheat and the other on an alternate host,
barberry (Berberis vulgaris).
New races of wheat rust can also arise through mutation in the
absence of sexual reproduction.

5. Heterothallism Vs homothallism
 The term heterothallism is used to designate when
there is the occurrence within a given species of
two kinds of individuals, each self-sterile.
 In this situation, intermycelial reaction is required
for sexual fusion, irrespective of the presence or
absence of differentiated gametangia. Eg Stem rust
(Puccinia graminis F.sp. Tritici)
• Conversely, the term homothallism is used when a
single individual that originates from a single spore
forms morphologically similar male and female
gametangia and is able to complete sexual cycle,
i.e., it is self-fertile and sexually self-sufficient.
eg. Rhizopus sexualis

6. Justification or Rationale
• The stem rust disease is one of the major yield limiting factors and
the current threat to Ethiopian wheat production due to the detection
of new physiological race, TTKS(Ug99) against Sr31 resistant gene
in East Africa.
• Ethiopia is one of the hot spot areas for the development of wheat
stem rust complex.
• In Ethiopia various efforts have been made to study different aspects
of wheat- Puccinia graminis f.sp. tritici (Pgt) interactions
• However, the variation in the resistance spectra of wheat varieties
against stem rust and its epidemiological implication such as
selection pressure on the pathogen was overlooked
• As a result, newly released varieties become out of production due to
stem rust attack even before being widely adopted
• A combination of variation assessment in Pgt virulence and/or wheat
varieties resistance spectra provides useful information to device a
better breeding and disease management strategy to use released
varieties in wheat production
• However, to this date, no recorded information is available about
resistance spectra of Ethiopian wheat cultivars to stem rust.

7. Objectives
• To assess the extent of variation in the resistance
spectra of Ethiopian hexaploid and tetraploid wheat
varieties and accessions against stem rust
• To assess epidemiological implication of the
variation as affected by pathogen evolution

8. Treatments
Three categories of wheat including
• Fifteen bread (Enkoy, Mitike, Kubsa,Galama, Tusie,
Megal, Katar, Shina, Hawi, Simba, Dodota, Sirbo,
KBG-01, Bobitcho and HAR-2504)
• Fourteen durum(Ardento, Gerado, Cocorit-71, LD-
375, Bichena, Quamy, Asasa, Ginchi, Ude, Dz-966-SY,
Dz-1935, Dz-95457, Dz-293-2DzR, Dz-1052) and
• Ten emmer wheat varieties and accessions(Sinana-
01,
• 33 stem rust differential lines?????
• Susceptible Bread wheat variety ‘Morocco’ used as
a control
• Eighteen characterized stem rust races

9. Major methods of study
Testing Materials
• Three categories of wheat including :15 bread,
14 durum and 10 emmer wheat varieties and
accessions were utilized in the study,
Susceptible bread wheat variety ‘Morocco’ used
as a control
• Eighteen stem rust races that were
characterized and maintained according to the
method of Naod B.,2004 at KARC, Ethiopia
were used. The study was conducted at KARC
in 2004

10. Major methods of study…
Multipathotype test
• Six seeds of each variety and accession were planted in 9-cm
diameter plastic pots to raise the seedlings
• The plastic pots were filled with a mixture of soil, compost
and sand in the ratio of 1:1:1 (v/v)
• The spore suspensions were prepared by mixing of
urediospores of each race in FC-40 mineral oil in a separate
beaker
• The spore concentration was adjusted to 4x106 spores/ml
• Seven days seedlings of each variety and accessions were
then inoculated by spraying the spore suspensions with and
atomizer
• Inoculated seedlings were incubated in a growth chamber at
20 + 2oc, 98 RH for 24 dark hours.
• The Seedlings were then transferred to the green house
benches until disease scoring was done

11. Major methods of study…
Disease assessments
• Disease assessments were conducted twice at 12 and 14 days after
inoculation according to the method by Stackman et al (1962) that
is infections types (ITS) of each seedling were scored by using a 0
to 4 seedling scoring scale (0= Immune- Nearly immune, 1=Very
resistant, 2= Moderately resistant, X = hetrogeneous,
3=Moderately Susceptible, 4= Susceptible)
Data Summary and analysis
• Infection types from 0-2 were summarized as resistance and 3-4 as
susceptible
• For ease of analysis, low infection types(LITs) were recorded as 1
and high infection types(HITs) as 0 (Mcvay,1991).
• The number of LITs exhibited by each genotype for all races was
counted and low infection type frequency (LITf) value was
assigned for each genotype.
• Data were interpreted based on the facts and assumptions of
gene-for-gene concept (Flor,1946; 1971).

12. Experimental Design
• Simple non-replicated design and the trial was
repeated twice was utilized

13. Data collected
• Disease assessments were conducted twice at 12
and 14 days after inoculation according to the
method by Stackman et al (1962) that is infections
types (ITS) of each seedling were scored by using a
0 to 4 seedling scoring scale (0= Immune- Nearly
immune, 1=Very resistant, 2= Moderately resistant,
X = hetrogeneous, 3=Moderately Susceptible, 4=
Susceptible)

14. How to meet the objectives set
• Multipathotype testing that involves the use of a large
number of races and differential lines is useful to study
and understand different aspects of host-pathogen
interations
• The success of multipathotype testing in describing of
the extent of variation in disease resistance depends
largely on the number and genotype of the pathogen
isolates used in the study(Burdon,1987).
• Accordingly, the authors believe that the 18 races in
this study were found adequate to show the variability
in the resistance spectrum of the 39 wheat varieties
and accessions.
• The results obtained give a clue on the epidemiological
and practical significance of knowing the resistance
spectrum of wheat varieties against stem rust

15. Major findings
• KBG-01, Bobitcho and Megal bread wheat varieties were found
resistant to all the 18 races of Puccinia graminis f.sp. tritici.
These varieties have the widest resistance spectra (LITf=18)
among the tested bread wheat varieties.
• The recorded wide resistance spectra on these bread wheat
varieties may be a favourable host factor for recently observed
susceptibility of Bobitcho and Megal varieties under field
conditions.
• The other bread wheat varieties such as Shina, Galma and
Katar that showed resistance to 8 races out 18 were found to
have narrow resistance spectra (LITf=8)
• The two widely grown Bread wheat cultivars Galama and
Kubsa also exibited narrow resistance spectra. Galama is
resistance to 8 races and Kubsa to 10 races out 18 races under
assessment.

16. Major findings …
• Among the tested 15 durum wheat varieties Ude was
the only variety with widest resistance spectra
(LITf=18) while the old semi dwarf variety Gerado
exhbited the narrowest resistance spectra (LITf=7).
• The three advanced lines of durum wheat Dz-966-SY,
Dz-995457, Dz-293-2DZR showed a resistance
spectra of 14, 15 and 17 respectively.
• The highest range of variation in the resistance
spectrum was also observed among durum wheat
genotypes.

17. Major findings …
• In the tested 10 Emmer wheat genotypes the widest
resistance spectra (LITf=16) was observed in ACC-
213155-2, Sinana-01 showed resistance to 14 races
(LITf=14) , while ACC- 212883-2 showed the
narrowest resistance spectrum (LITf=11).
• The range of variation in the resistance spectrum was
found minimum in the emmer lines as compared to the
range of variation observed in durum and Bread wheat
varieties and accessions.

18. Figure 1. Range of variation in the
resistance spectra of wheat varieties

19. Major findings’ implications
• The results obtained give a clue on the epidemiological
and practical significance of knowing the resistance
spectrum of wheat varieties against stem rust
• Varieties with high LITf of 18 such as KBG-01, Bobitcho,
Megal and Ude would have been expected to fit and
maintain their resistance in stem rust population that
contains races with wide resistance spectrum.
• However, in reality where a dynamic system of host-
pathogen interaction exists, LITf values could be used
to explain epidemiological concepts such as Boom-
Bust cycle, directional and stabilizing selection(Vander
Plank, 1982) and Magnitude of fitness(Groth, 1984)
• The level of resistance spectrum expressed as LITf was
also found to be a quantitative clue of the selection
pressure in host pathogen interaction.

20. Making terms clear
• A differential interaction between host varieties and
pathogenic races allows the host to exert direct selection
pressure on races of the pathogen; there is directional
selection of pathogenic races in favor of those compatible
with the host.
• In the absence of a differential interaction between host and
pathogen, any selective advantage or disadvantage a
pathogenic race may have is necessarily only indirectly and
remotely related to the variety of the host.
• A differential interaction between host varieties and
pathogenic races is the source of the boom and bust cycles
for which vertical resistance is notorious.
• Conversely, the absence of a differential interaction is the
source of the stability that one associates with horizontal
resistance.

21. Major findings’ implications…
• the variation in frequency distribution of infection types in a
population provides clues concerning the nature and extent of
selection pressure placed on individual plant populations by fungal
plant pathogens.
• Based on this premise, the epidemiological meaning and logical
relationship between the observed LITf of the varieties and the
extent of selection pressure they could exert on the pathogen
could be postulated as follows:
• In cases where gene-for-gene interaction works , varieties with
highest LITf values ( broad resistance spectrum) exerts higher
selection pressure on the pathogen than those varieties with
lower LITf values( narrow resistance spectrum)
• Varieties with broad resistance spectrum cause greater pathogen
co-evolution than those varieties with lower resistance spectrum.
• Thus varieties such as KBG-01, Bobitcho, Megal and Ude are the
most important to dictate the evolution of new virulent races in
Ethiopian stem rust population than Shina, Galema, Katar and
Gerado.

22. Major findings’ implications…
• Introduction of Broad spectrum resistance varieties
(with higher LITf possessing higher magnitude of
fitness)in to the host-pathogen system could exert
more selection pressure and disturb the
equilibrium as explained in the concepts of Boom-
Bust cycle,
• Hence the variation in LITf values could be taken as
quantitative value that signify the differential levels
of selection pressures exerted by different varieties
on the pathogen population which consequently
results in the shorter life span of released varieties

23. Major findings’ implications…
• Cultivated emmer wheat accessions were found to be
good source of stem rust resistance (Naod et al., 2007)
and hence they can be alternative source of resistance
to wheat breeding program.
• LITf values of each variety gave clues to the
epidemiological relationship between LITf values and
pathogen coevolution.
• In case where gene-for-gene interaction works Varieties
with highest LITf values exert higher selection pressure
on the pathogen population than those varieties with
lower LITf values and consequent upon this hypothesis
the importance of assessing LITf values of each variety
in designing and implimenting vertical resistance
management strategy

24. Recommendations of the work
• Regional gene deployment, gene pyramiding,
multilines, mixtures and varietal diversity are strategies
that are designed to lengthen the duration of vertical
resistance genes(Van Der Plank, 1982; Knot, 1989).
• Despite all their limitations these strategies remains the
best at present as long as breeding programs could not
develop varieties with non-specific resistance genes as
required(Roelefs et al.,1992).
• Cultivated emmer wheat accessions were found to be
good source of stem rust resistance (Naod et al., 2007)
and hence they can be alternative source of resistance
to wheat breeding program.

25. Future research directions
• Development of Varieties with non-specific resistance
• Regional gene deployment, gene pyramiding,
multilines, mixtures and varietal diversity to lengthen
the duration of vertical resistance genes
• Screening of wheat varieties for disease resistance
should focus on selecting varieties with horizontal
resistance rather than selecting only those with vertical
resistance free from disease
• Use of Emmer wheat varieties as a source of resistance
gene for stem rust

26. Limitations of the study
• The number of genotypes of different wheat might
not be sufficient to compare them as a separate
entity, and suggest their differential role in the
Ethiopian stem rust epidemiology.
• However, important difference observed among the
three wheat species in Ethiopia in terms of their
origin, methods of development (selection or
crossing) of a variety and spatial distribution (extent
of cultivation).These difference among the wheat
varieties could affect the host-pathogen co-
evolution, population equilibrium and the rate of
stem rust pathogen spread and evolution of new
races beside the LITf values

27. Limitations ….
• The authors focused on seedling resistance irrespective of other stages of
growth which favour the occurrence of stem rust
• Lack of replication and design
• Old literatures utilized in both in the background and in the validation of
the result
• Inability of the study to see the host-pathogen dynamic under field
condition
• All wheat varieties in the country were not included in the trial
• On their Abstract the authors indicated that they utilized 33 stem rust
differential lines in addition to the three wheat varieties and accessions
however no data on the effects of races on differentials

28. • Stabilizing selection is a relatively old concept of classical
evolution which explains why species under natural conditions
remain more or less constant and relatively unchanging.
• Present-day theorists of evolution have considered stabilizing
selection to be the mechanism whereby a species maintains its
status quo within environmental extremes
• Stabilizing selection is the opposite of directional selection or
adaptation.
• the quality of a resistance gene in the host determines the fitness
of the matching virulence gene in the parasite to survive when
this virulence is unnecessary; and
• reciprocally the fitness of the virulence gene to survive when it is
unnecessary determines the quality of the matching resistance
gene as judged by the protection it can give to the host.