Scald

Rhynchosporium secalis

Biology

Scald is the major foliar disease of barley in the wetter regions of the prairies, particularly the parkland zone of Alberta. This disease can also affect rye and some grasses.

The scald fungus overwinters on barley residue, grain and grasses, particularly species of Bromus. In spring, spores are produced mainly from barley residue left on the soil surface from the previous year. Spores produced on infected leaves are transported to other barley plants by rain drops and wind. Scald can attack a barley plant at any time but levels of infection are usually most severe just before and during heading. The significance of scald-infected seed as a source of disease spread in barley is not well understood.

The disease is favored by cool (12-20 degrees Celsius), humid weather and dense crop canopies where leaves remain wet for prolonged periods.

Damage Description

Scald is primarily a foliar disease but is also found on leaf sheaths and glumes. Large water-soaked, grey-green spots appear on the leaves, rapidly dry out and become bleached with brown margins. Spots often join and kill the entire leaf.

Yield losses in Alberta have been calculated at 2.4 per cent, although losses in particular fields may exceed 25 per cent. Losses are due to a decrease in photosynthetic area on the flag and second leaves resulting in reduced seed weight. Severe scald infections can cause damage that resembles black point but is not confined to the embryo end.

In general there is a direct relationship between the amount of disease present on upper leaves of cereals and the resulting loss in grain yield. The top two leaves of the cereal plant supply most of the energy required to produce well-filled grain. This principal can be used to predict potential yield losses at the milky ripe growth stage.

If scald appears on the upper leaves and sheaths in mid-July, anticipate considerable yield loss. If the scald does not appear until early to mid-August, disease levels may be heavy but actual losses from this disease will be considerably reduced since the grains are well filled by this time.

Diagnosis

Examine the crops at the milky ripe stage (see Feekes scale). Assess no less than 25 main tillers selected at random along two diagonals from one corner to the opposite corner of the field. Assess the percentage of first and second leaves affected. Take the average of the first leaf and the second leaf and apply it to the following formula. This will give you a reasonable estimate of expected crop loss from scald.

% yield loss = 2/3 x % area of flag leaf infected + 1/2 x % area of second leaf infected / 2

• Use resistant cultivars. See Varieties of Cereal and Oilseed Crops for Alberta
  Turn under or deep till surface barley residue. This reduces disease levels when barley follows barley.
• Seed early with an early maturing variety to miss the major build-up of disease that hits later-sown crops and later maturing varieties.
• Use a crop rotation of at least one year with non-host crops such as other cereals or canola.
• Use resistant cultivars.
• Apply a foliar fungicide. Tilt 250E/Bumper 418EC (propiconazole) and Headline EC (pyraclostrobin) are registered on barley in Canada for control of scald and net blotch.
• Baytan 30 (triadimenol) applied as a seed treatment will provide suppression of the seed-borne phase of scald.