Infection

The infection or penetration of a plant host by infectious propagules is also determined by specific environmental conditions. Fungal pathogens usually require high relative humidity or even free water for infection. The infection process is limited by the duration of surface wetness or high humidity in most terrestrial environments (Magarey et al., 2005). Various components of climate change are likely to affect the level and duration of humidity in the environment of pathogens, including temperature and rainfall, and CO2 concentration through its effect on plant growth and on the canopy microclimate. If a cropping system is subjected to a dry environment, conditions become less favourable for several foliar diseases, such as powdery mildew of cereals caused by Blumeria graminis, which requires high relative humidity to penetrate host tissues and colonize the leaf. Elevated concentrations of CO2 affect infection of barley by B. graminis by increasing mobilization of assimilates and limiting the penetration of the pathogen (Hibberd et al., 1996). Similarly, variation in the distribution and predominance of pathogens resulting in Fusarium head blight (FHB) of wheat or scab, caused by several species of Fusarium and Microdochium, is another example of how climatic factors, particularly temperature and moisture, determine the comparative abundance of these fungi on infected wheat ears. Scab is most severe in warm and wet conditions at anthesis, and Fusarium gramin-earum (teleomorph Gibberella zeae) is the predominant species in these areas, although FHB incidence has increased in cooler areas (Xu et al., 2008), suggesting an evolution in the factors influencing the disease cycle. Whereas Fusarium poae is associated with relatively drier and warmer conditions, Fusarium avenaceum and Fusarium culmorum are associated with areas where conditions are cooler and humid. Thus, the environment affects the infection and colonization processes in different ways, which could lead to shifts in the comparative abundance of the species (Garrett et al., 2006). This could eventually affect the spectrum of predominating mycotoxins generated by species causing FHB, which is a concern for food and feed safety (Jennings et al., 2004). Similarly, cropping practices such as zero and minimum tillage could be associated with higher G. zeae colonization in areas where wheat is grown after maize, an alternate host for the fungus, which highlights the role of survival capacity in potential epidemics (Bateman et al., 2007). In Europe, the occurrence of Phaeosphaeria nodorum causing Septoria nodorum blotch in wheat has become less important since the late 1970s compared with the increased prevalence of Mycosphaerella graminicola, the causal agent of Septoria tritici blotch. Even if changes in varieties and fungicide use partly explain the higher prevalence of M. graminicola over P. nodorum in recent years, the long-term reduction in SO2 levels in the air is correlated with the relative occurrence of both fungi and explains a shift in their respective incidence (Bearchell et al., 2005). Recently, the deuteromycete Ramularia

Table 4.1. Effect of climate and human-induced activities on disease cycle components in selected food crop pathosystems.

Disease cycle

Crop

Effect of climate and human-induced

component

Pathogen/vectors

Disease

affected

Observation

activities

References

Survival

Fusarium graminearum,

Fusarium head

Wheat

Fusarium head blight

Maize grown at higher latitudes; over

Bateman et al.

Fusarium culmorum

blight

severity increase

wintering of inoculum on previous

(2007)

crop residues (maize) under zero

tillage

Puccinia triticina

Leaf rust

Wheat

Yield losses increased in

Over-wintering of inoculum

Eversmeyer and

some areas

Kramer (2000)

Rhopalosiphum padi,

Aphid vectors of

Oats,

More BYDV

Vector overwintering is favoured by mild

Malmström and Field

Sitobion avenae,

barley yellow

barley,

winters; CO2 increases root biomass

(1997), Fabre et al.

Metopolophium

dwarf virus

wheat

and water-use efficiency of infected

(2005), Chancellor

dirhodum

(BYDV)

plants (virus reservoirs)

and Kubiriba (2006)

Infection

Blumeria graminis

Barley powdery

Barley

Reduced penetration of

Dry air environment; elevated CO2

Hibberd et al. (1996),

mildew

the fungus

concentrations mobilize assimilates

Jahn et al. (1996)

and plant response

Cochliobolus sativus

Spot blotch

Wheat

More wheat areas

Rising temperatures, particularly night

Sharma and

affected and increased

temperatures, increase host

Duveiller (2004),

severity

susceptibility

Sharma et al.

(2007)

F. culmorum

Fusarium head

Wheat

Incidence and severity

Cool and humid environment favours

Jennings et al.

blight

disease

(2004), Xu et al.

(2008)

F. graminearum

Fusarium head

Wheat

Incidence and severity

Warm and wet environment at anthesis

Jennings et al.

blight

favours disease

(2004), Xu et al.

(2008)

Fusarium

Dryland root rots

Wheat

Prevalence in dryland

Drought-stress affected areas

Duveiller et al. (2007)

pseudograminearum,

and nematodes

areas

increasing; optimum irrigation less

F. culmorum,

available

C. sativus and

nematodes

(Heterodera spp.,

Pratylenchus spp.)

Mycosphaerella graminicola

Septoria tritici Wheat blotch

Latency

Phaeosphaeria nodorum Septoria nodorum Wheat blotch

Ramularia colo-cygni Ramularia leaf Barley spot

Bemisia tabaci (whitefly) Vector of cassava Cassava mosaic virus

Cicadulina mbila and other leafhoppers

Vector of sweet potato chlorotic stunt virus Vectors of maize streak virus

Sweet potato

Maize

Phytophtora infestans Potato late blight Potato

Puccinia triticina

Leaf rust

Wheat

Dispersal

B. graminis

Powdery mildew Barley wheat

M. graminicola P. infestans

Septoria leaf Wheat blotch

Potato leaf blight Wheat

Puccinia graminis f. sp. Stem rust tritici

Wheat

Prevalence and severity increased in last decades Prevalence decreased in

Western Europe Emerging disease

Disease prevalence associated with vector multiplication Disease prevalence associated with vector multiplication Disease prevalence associated with vector multiplication Model predicts fungicide needed for longer period Increased disease severity Increasing incidence in new areas

Spore dispersal favoured

Severity increased in rainy years

Severity increased in rainy years Ug99 dispersal progressing to Iran

Reduction in S02 in the air in last decades; rainfall patterns

Reduction in S02 in the air in last decades

Effect on host physiology influencing susceptibility to toxin

Reduction in generation time of the vector

Jahn etal. (1996), Bearchell et al. (2005) Bearchell etal.

(2005) Schutzendubel et al. (2008)

Chancellor and Kubiriba (2006)

Reduction in generation time of the vector

Chancellor and Kubiriba (2006)

Reduction in generation time of the vector

Chancellor and Kubiriba (2006)

1-3°C temperature increase accelerates pathogen multiplication; longer epidemics Warmer and wetter growing seasons

Kaukoranta (1996), Boland et al. (2004) Baker et al. (2005)

Reduction in generation time

FAO (2008)

Dry air and warm temperature favouring Jahn et al. (1996), spore spread

Rain splashes and rainfall patterns changed Rainfall patterns changing

Wind; outstanding storms

Chancellor and Kubiriba (2006) Jahn etal. (1996)

Baker etal. (2005)

Hodson etal. (2009)

colo-cygni, a pertotrophic fungus producing a toxin that leads to leaf infection at a late growth stage, has gained increasing importance in Europe as the causal agent of a new leaf spot disease in barley, Ramularia leaf spot. The physiological status of the host appears to govern the susceptibility of winter barley to this pathogen (Schutzendubel et al., 2008). In southern Asia, spot blotch of wheat caused by Cochliobolus sativus is more severe under stress conditions, such as heat or poor soil quality, and is therefore highly dependent on plant physiology and growth stage (Sharma and Duveiller, 2004). A 6-year study at multiple sites has shown that disease severity increased with rising temperatures, particularly night temperatures, after anthe-sis, suggesting that more wheat growing areas will become affected by spot blotch, along with heat stress affecting more regions (Sharma et al., 2007; Ortiz et al., 2008).

Soilborne pathogens, including dryland root rot and cereal nematodes, have a global distribution and cause yield losses in rainfed regions where cereals dominate the cropping system and in irrigated areas where water supply or rainfall might not always be adequate, exposing the crops to water stress and potential damage by these pathogens (Duveiller et al., 2007). As climate change is expected to increase the number of drought-stress affected areas around the world, the severity of root diseases such as common root rot (C. sativus), foot rot induced by several Fusarium pathogens, as well as nema-tode problems, will increase when irrigation becomes limited, as illustrated by the prevalence of these diseases in rainfed wheat-based cropping systems in northern Africa and western Asia.

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