Diseases occur when a susceptible host is exposed to a virulent pathogen under favourable environmental conditions.
Control is best achieved by knowledge of the pathogens involved and manipulation of the interacting factors. Little can be done to modify the environment but growers can minimise the risk of diseases by sowing resistant varieties and adopting practices to reduce inoculum.
Resistant varieties provide the easiest and most effective option, yet few varieties possess adequate resistance to all major diseases. Alternative strategies are therefore required to reduce risk.
Rotate barley crops with non-hosts such as wheat (to control foliar pathogens), legumes or summer crops; avoid sowing barley on barley and maintain clean fallows. Sowing out of season favours disease development and can build up inoculum early in the season.
The main foliar diseases of barley are leaf spots, rusts and powdery mildew. Leaf spots and powdery mildew occur over season on crop residues while rusts (and to a lesser extent, powdery mildew) require living plants to carry them from one season to the next.
Growers should assess the disease risk of individual paddocks before sowing. Consider the recent history of a paddock, the incidence of diseases in recent barley crops and the amount of infected stubble in the target and neighbouring paddocks. Infected stubble can usually be recognised by the presence of small black 'pimples' on the straw.
Head and root diseases
Crown rot (Fusarium psuedograminearum)
Common in winter cereals. It is soil-borne and can be carried over from one season to the next on barley and wheat stubble. A barley variety with the same rating as a wheat variety will not suffer the same level of yield loss. However, yield losses as high as 28% have been recorded. Rotation with chickpeas or summer crops is currently the best method of controlling crown rot as there are no varieties which possess adequate field resistance.
Covered smut (Ustilago hordei)
A photo of covered smut in heads of barley
Recognised by the appearance of masses of spores (fungal seeds) enclosed in a semi-persistent membrane that covers the smutted spikelets. This is in contrast to loose smut of barley where there is no membrane and the head is a ragged mass of spores. Covered smut survives as teliospores (resting spores) on seed or in infested soil.
Spores geminate when the soil temperature range is 14-25ºC and soil moisture is low. Infection occurs as the seedlings emerge from the sprouting seed. The fungus enters the young seedling and grows within the growing point of the developing barley plant.
The greater the amount of infection on this tissue, the greater the number of tillers per plant infected.
In smut-infected plants, a smutted head emerges rather than a flowering barley head. The membrane covering the smutted tissues keeps the smut spores (teliospores) from being dispersed until harvest.
At harvest thin membranes that cover the sori are ruptured and the spores are released and mix with healthy seed or fall to the ground. When the membrane ruptures prior to harvest, spores are blown to healthy seed. Teliospores contaminate the exterior portion of the seed, especially the crevice.
Spores which land between the kernel and the hull germinate when adequate moisture is present. The mycelium arising from these spores goes dormant after it becomes established. Spores on the seed and dormant mycelium associated with hulls are responsible for most of the infection in the subsequent growing season.
If growers have any concerns about infection of their seed they should look at accessing seed from a clean source. The cost of treating seed is minimal in comparison to losing production or not being able to market the grain. As the disease can be soil borne as well as seed borne it is recommended that planting seed be treated every year regardless of cleanliness of the seed source.
Seed obtained through reputable seed companies should be treated, however farmer kept or farmer traded seed may not be treated and growers need to be wary.
Seed treatments are available for control of:
- covered and loose smuts
- seed borne net blotch (net form)
- seed borne spot blotch
- powdery mildew (up to eight weeks protection).
Common root rot (Cochliobolus sativus)
Also soil-borne. It is widespread in barley crops and may cause yield losses of up to 15%. As there are no varieties resistant to this disease, rotation with summer crops or winter legumes is the best method of control.
See the Barley variety characteristics table for information on variety resistance to diseases.
Black point in barley
Black point is a dark discolouration at the germ end of otherwise healthy barley grain. Yellowing of the germ end of the grain is probably also a variation of black point discolouration. The discolouration occurs in the glume tissue (palea and lemma) which remains adhered to the outer seed coat.
The conditions that cause high levels of black point in wheat often do not cause any discolouration in barley. The precise nature and timing of the environmental stresses that induce black point symptoms are not fully understood, however, yield, humidity and temperature are likely to be involved.
Black point causes a slight reduction in the rate of germination but without a significant effect on the malting process. However, in combination with shrivelling, field mould or disease, grain health may be affected and germination rates reduced.
Root-lesion Nematode (Pratylenchus thornei)
Barley is tolerant to root-lesion nematode infection and is therefore a good winter cropping option in infested areas. Although barley is more tolerant than many wheat varieties and most varieties are resistant to very resistant, nematode numbers in the soil may still increase under barley cropping. Rotations using resistant crops are encouraged.
The following table outlines the common foliar diseases and the basic information on survival dispersal infection times and control options.
|Disease||Survival mode||Optimal conditions||Dispersal||Growth stage when infected (Zadoks)||Pot. yield loss (%)||Control options|
|Leaf rust||Living barley plants||15-22ºC;
tillering to maturity
|30+||Prevent green bridge
|Stem rust||Living wheat, barley, rye, or rough wheat grass||20-30ºC
booting to maturity
|50+||Prevent green bridge
|Net blotch||Stubble, seed, volunteer plants||15-25ºC
emergence to maturity
|Spot form net blotch||Stubble, volunteer plants||15-25ºC
first leaf to maturity
|Spot blotch||Stubble, seed, soil||20-30ºC
emergence to maturity
|Powdery Mildew||Stubble, volunteer plants||15-22ºC
first leaf to maturity
Leaf rust (Puccinia hordei) and stem rust (Puccinia graminis tritici, secalis and tritici x secalis)
Traditionally the major airborne diseases of barley in Queensland. More likely to occur in wetter years or higher rainfall areas. Both can cause significant yield loss and quality downgrading. Grain yield can be reduced by up to 50% by stem rust and around 30% by leaf rust. As stem rust may infect barley and wheat, an epidemic could put both crops at risk.
The best protection from either disease is to plant resistant varieties and avoid planting very early or very late in the season. Barley occupies approximately 25% of the winter cereal area. As long as resistance levels of all winter cereals (especially wheat) remain high, it is not expected that major epidemics of stem rust will occur in barley. In emergencies timely application of fungicides can be effective.
Net blotch (Pyrenophora teres)
Has become the most significant disease of barley in the region and is likely to be a problem in wetter years and in stubble-retained situations.
It occurs in two forms: net form of net blotch (P teres f. teres) and spot form of net blotch (P. teres f. maculata).
The net form produces dark brown to black stripes on leaves and leaf sheaths of older plants and gives a characteristic netting pattern in juvenile leaves. The spot form of net blotch produces dark brown, round to elliptical spots on leaves and leaf sheaths that are often surrounded by yellowing.
High levels of either disease will kill leaves prematurely which may cause yield losses in excess of 30%. Growers are advised to avoid planting barley on barley where stubble is retained, as stubble-borne spores are the main source of infection for the new crop.
The net form of net blotch may be seed borne and grain from heavily diseased crops should not be retained for planting.
Net form of net blotch
Spot form of net blotch
Spot blotch (Cochliobolus sativus)
Favoured by warm wet conditions and is promoted by stubble retention. It can be seed-borne. Leaf symptoms are almost identical to the spot form of net blotch. Spot blotch may also cause discolouration of grains. This disease is more likely to be a problem in sub-coastal areas. Popular varieties are susceptible.
Spot blotch of barley
Barley Grass Stripe Rust (Puccinia striiformis)
Will infect some varieties of cultivated barley. Most current varieties grown in Queensland have good resistance and the disease is unlikely to be a major problem in the northern region in the near future.
Powdery mildew (Blumeria graminis hordei)
Often present in susceptible varieties, but generally causes only relatively small yield loss (usually less than 10%). Some seed treatments can give good early season control of powdery mildew but these may also shorten coleoptile length and cause emergence problems. Resistant varieties are the best means of control.
Some varieties may appear susceptible at the seedling stage but develop resistance to mildew during elongation. Where this occurs lower leaves of infected plants may take on a blotchy appearance which can be confused with other blotches caused by other diseases. Where powdery mildew is responsible fine mycelium is usually evident on the older leaves.
Powdery mildew on barley