Factors Effecting Disease Damage

A virulent pathogen (disease causing organism), a susceptible host and a favourable environment must all be present for disease outbreak to occur. There are differences in the likelihood of damage among the foliar diseases of wheat and barley. Some diseases are prevalent, but only rarely cause significant damage, whereas other diseases are rare, yet can cause severe yield loss in certain seasons and rainfall zones, (Table 2.1). Therefore, it is important to accurately identify the disease when making management decisions.

For help with disease identification refer to the specific DEDJTR AgNotes for each disease.

Table 2.1 Likelihood and potential severity of major foliar diseases of wheat and barley, and the suggested minimum level of resistance in varieties in three rainfall regions in Victoria – refer table 2.3 for towns in these three rainfall zones

Crop Disease Pathogen Likelihood1

Potential Damage2

Suggested minimum Level of Resistance3

  Environment Annual Rainfall (mm)  Environment Annual Rainfall (mm)  Environment Annual Rainfall (mm) 
  <350 350-500 >500 <350 350-500 >500 <350 350-500 >500
Wheat Leaf rust Puccinia triticina 2 2 3 2 2 3 MS MS MR
Stripe rust Puccinia striiformis 2 3 3 2 3 3 MS MR-MS MR
Stem rust Puccinia graminis 1 2 2 3 3 3 S MS MR
Yellow leaf spot Pyrenophora tritici-repentis 3 3 0 3 2 3 S S S
Septoria tritici Mycosphaerella graminicola 1 2 3 1 2 3 S MS MS
Septoria nodorum Phaeosphaeria nodorum 1 1 2 1 2 3 MS MS MS
Powdery mildew Blumeria graminis 2 2 2 1 2 3 S S S
Barley Leaf rust Puccinia hordei 2 3 3 2 2 2 S S MS
Spot form net blotch Pyrenophora teres f. maculata 3 3 3 2 2 3 S S MS
Net form net blotch Pyrenophora teres f. teres 3 3 3 3 3 3 MS MS MR
Scald Rhynchosporium secalis 2 3 3 3 3 3 S MS MR
Powdery mildew Blumeria graminis 2 2 2 2 2 3 S MS MR

 

1 The likelihood of suitable conditions for crop disease: 0 = not known to occur, 1 = unlikely (less than 1 in 10 years), 2 = somewhat likely (between 1 in 10 to one in 5 years), 3 very likely (more often than 1 in 4 years)

2 The potential damage should suitable conditions for disease occur: 1 = Yield loss unlikely, 2 = Yield loss up to 50%, 3 = Total crop loss possible

3 The suggested minimum level of disease resistance required in varieties to avoid yield loss in most seasons (see Table 2.2 for definitions)

 

Crop varieties are rated for their level of resistance to the major diseases, (Table 2.2). These ratings can change following change in the virulence of the pathogen or a new pathogen introduction into Australia or from interstate. Hence, it is important to always use the latest cereal disease ratings (refer to the Victorian Cereal Diseases Guide (2105) for current information). The more resistant the variety, the less chance it will need additional protection from fungicides for that disease. Resistant varieties should be the first option in disease management, but adequate resistance to all important diseases is not often available in all varieties.

In Victoria, there are three general environments, based on average annual rainfall, where cereals are grown; long-term climatic conditions typical of towns in these rainfall zones are indicated in Table 2.3. Each disease needs specific environmental conditions to develop, but in general fungal diseases require a moist environment for sporulation and infection to occur, (Table 2.4). Thus the higher rainfall areas are more likely to suffer from foliar disease, and greater levels of resistance are required for adequate control, (Table 2.4). It helps to understand the environment when developing disease management strategies.

 

Table 2.2 Ratings of varietal resistance and what they mean

DISEASE RATING SYMBOL DESCRIPTION
Resistant R Disease symptoms may be found at very low levels following close crop inspection. Generally, no management is required even in instances of high disease pressure. Monitor crops for breakdown in resistance.

 

Resistant – Moderately Resistant R–MR Disease symptoms may be found at low levels following close crop inspection. Generally, no management decisions will be required. Monitor crops for breakdown in resistance
Moderately Resistant MR Disease symptoms may be observed, but generally no management decisions will be required. However, under severe disease pressure active management may be necessary. Monitor crops for disease.
Moderately Resistant – Moderately Susceptible

 

MR–MS

 

Disease symptoms may be observed, but generally no management decisions will be required.Preventative action may be economic in years with high disease pressure. Monitor crops.

 

Moderately Susceptible MS In seasons conducive to disease, symptoms will be detected during crop inspections. If the disease appears early in the season then preventative action may be necessary. Crop losses of 15% or more can occur in severe cases. Monitor crops.
Moderately Susceptible– Susceptible MS–S In seasons conducive to disease, symptoms will be detected during crop inspections. When varieties drop below this minimum level of resistance there can be a build-up of disease. Still a useful level of resistance. Management decisions could be required to reduce disease. Monitor crops regularly.
Susceptible S Disease symptoms will be easily found during crop inspections. Management decisions will be required to reduce disease. Expect yield loss in the rangeof 15-50% without management. Monitor crops regularly.
Susceptible – Very Susceptible S–VS Disease symptoms will be easily found during crop inspections. Crop loss can be greater than 50% in the absence of management. It is not recommended to grow varieties with low resistance if the disease is common to the area. Monitor crops regularly.
Very Susceptible VS Disease symptoms will be easily found during crop inspections. The use of this resistance level is not recommended if the disease is common in an area, as there can be total crop loss. Active management with fungicides is not generally considered economic.

Table 2.3 Long term weather data for Ouyen (Mallee), Horsham (Wimmera) and Hamilton (Southwest Vic)A

Parameter (years of data)

Annual Rainfall 

(January to December)

Growing Season Rainfall 

(April through November)

  Ouyen Horsham Hamilton Ouyen Horsham Hamilton
Mean daily maximum temperature - °C (95.3 years) 24 22 19 20 18 16
Mean daily minimum temperature - °C (95.3 years) 10 8 8 7 6 7
Mean 9am relative humidity – % (73.5 years) 67 71 75 73 77 80
Mean 3pm relative humidity – % (72.3 years) 42 48 56 47 55 62
Mean total rainfall – mm (111.5 years) 333 449 689 244 350 537
Median (5th decile) total rainfall - mm (108 years) 339 441 675 209 317 513

A Source: Bureau of Meteorology

Table 2.4 Conditions needed for development of the major pathogens of wheat and barley

CROP DISEASE PATHOGEN TEMPERATURE C LEAF
WETNESS1
(HOURS)
LATENT
PERIOD2
(DAYS)
SPORE
DISPERSAL
PRIMARY
INOCULUM
SOURCE
      RANGE OPTIMAL        
WHEAT Leaf rust Puccinia triticina 10-35 15-25 3 7-10 Wind Volunteer wheat
Stripe rust Puccinia striiformis 0-23 10-15 3 20-86 Wind Volunteer wheat
Stem rust Puccinia graminis 15-40 20-30 6-12 14 Wind Volunteer wheat
Yellow leaf spot Pyrenophora tritici-repentis 10-30 18-24 6-48   Wind Infected stubble
Septoria tritici blotch Mycosphaerella graminicola 3-35 15-20 6 10-20 Rain, Wind Infected stubble
Septoria nodorum blotch Stagonospora nodorum 4-35 20-27 6-16 10-20 Rain, Wind Infected stubble
Powdery mildew Blumeria graminis 1-30 15-22   7-10 Wind Volunteers, Crop Residues
BARLEY Leaf rust Puccinia hordei

 

15-22

 

 

Wind Volunteer barley
Spot form net blotch Pyrenophora teres f. maculata 8-33

 

    Wind Infected stubble
Net form net blotch Pyrenophora teres f. teres 5-30 15-25 5   Wind Infected stubble, Seed
Scald Rhynchosporium secalis 1-30 15-20 72 12-28 Rain, Wind Infected stubble
Powdery mildew Blumeria graminis   15-22   7-10 Wind Volunteers, Crop Residues

1 The longer the period of leaf wetness the greater the chance of successful infection.

2 The latent period is the amount of time after infection before symptoms are present. It generally decreases as temperature increases within the optimal range.

How Foliar Disease Reduces Yield and Quality

Crop Phenology - The Growth Stages