Applying potassium (K) to barley crops that are deficient in this nutrient can boost grain yields and reduce the impact of some foliar leaf diseases, such as powdery mildew and spot type net blotch.
Potassium is considered deficient at soil test levels of less than 50 mg/kg (ppm) in the top 10 cm.
Plant tissue testing for K early in the growing season can help protect all cereal crops and optimise fungicide use, especially in regions prone to leaf diseases and where K deficiency is known or suspected.
This year there have been early reports of powdery mildew in southern grain areas and latest research indicates commonly used barley varieties in WA are more susceptible to this disease than first thought.
Department of Agriculture and Food WA (DAFWA) South Coast trials from 2001-04, funded by GRDC, showed disease-susceptible barley and wheat crops deficient in K were more prone to powdery mildew and spot-type net blotch.
This project, led by DAFWA principal researcher Dr Ross Brennan, found K applications eight weeks after seeding to those crops on sandy soils could reduce leaf disease incidence and were profitable – provided there was enough nitrogen for plants to respond to the extra potassium.
HOW MUCH IS ENOUGH POTASSIUM?
As K inputs increased – from 0 to 40 kilograms/hectare – the percentage of diseased barley leaf area from spot-type net blotch and powdery mildew fell by about 30 per cent.
Maximum barley grain yield occurred with adequate K applications of 8-22 kg K/ha and where fungicides were used to control leaf diseases.
Researchers found no additional disease control or yield benefits from applying K to barley at higher rates (from 40 to 120 kg K/ha).
This project showed extra K fertiliser applications had no benefit in reducing rust incidence in barley or wheat.
There was no affect on barley or wheat grain protein from rate or type of K fertiliser, but protein percentage dropped marginally in line with fungicide application.
Percentage of screenings fell with extra K applications and the use of fungicides to control powdery mildew and spot-type net blotch.
The DAFWA trials were conducted in a medium rainfall zone where Colwell soil test K levels in the top 10 cm were less than 50 mg/kg (ppm) and nitrogen (urea) was applied at a rate of 90 kg/ha of N. Results are further illustrated in the figures below.
Figure 1. The percentage leaf area diseased (%LAD) response of barley to application of K fertiliser (kg K/ha) as sulphate or chloride with and without the application of fungicides to control powdery mildew. The two curves are significantly different.
Figure 2. The grain yields (kg/ha) response of barley to application of K fertiliser (kg K/ha) as sulphate or chloride with and without the application of fungicides to control powdery mildew. The two curves are statistically significantly different.
IMPORTANCE OF K SOURCE
Potassium chloride (KCI - 50%K, or muriate of potash) was generally more effective in reducing powdery mildew levels at lower rates (10-20 kg/ha) than potassium sulphate (K2SO4 - 41%K, or sulfate of potash).
In some of the K trials, double the amount of sulphate K – compared to chloride K - was needed to reach 90 per cent of maximum barley grain yield where crops were infected by powdery mildew.
The K trials indicated that in medium to high rainfall regions, 20-50 kg/ha of chloride K or 44-97 kg/ha of sulphate K are needed to maximise yields.
At this level, powdery mildew and other leaf disease incidence will fall, but not to the level of protection afforded by fungicides.
The two K sources used in these trials did not affect the incidence of spot-type net blotch in barley where this disease was present, or resulting grain yields.
INTEGRATED DISEASE MANAGEMENT
An integrated cereal leaf disease management system should include fungicide sprays in conjunction with any extra K fertiliser applications – the need for which can be determined by soil and plant tissue testing.
It is recommended that K fertiliser be applied six to eight weeks after barley crop emergence, when roots are developed enough to take up this extra nutrient and potential leaching losses are reduced.
DAFWA plant pathologist Dr Kith Jayasena is currently undertaking further research into foliar K applications and the impact on leaf disease and grain yields.
Ross Brennan, DAFWA Albany
08 98 928 474
Kith Jayasena, DAFWA Albany
08 98 928 477
DAFWA Farm Note 216: www.agric.wa.gov.au
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