PestFacts WA

Do you have patches of red and dying subterranean clover?

• Brookton
• Williams
• Mount Barker

Sub-clover with red leaves from a Brookton paddock. Photo courtesy of: DPIRD

A farmer has reported to the department that he is seeing patches of subterranean clover (sub-clover) turning red and dying on his Brookton property. This is a concern as it is reducing what limited pasture he currently has on offer to feed his sheep. The farmer did a quick survey of other farmers in the shire and red sub-clover appears to be widespread in the Brookton shire this year.

Another farmer has also reported red sub-clover on his property west of Williams. The red sub-clover is visible in paddocks that have had 3-4 consecutive years of sub-clover.

A patch of red and dying sub-clover. Photo courtesy of: Stuart Irwin (Farmer)

Farmer Stuart Irwin also currently has patches of sub-clover that are dead or very red in colour across his property that is located west of Mount Barker. Stuart says in some areas 100% of the sub-clover has died and this is the worst year for this occurrence. Stuart suspected that the clover had sub-clover red leaf virus (SCRLV) but virus testing by the department’s Diagnostic Laboratory Services (DDLS) Plant Pathology services revealed that the sub-clover plants did not have SCRLV and were more likely to be stressed.

What causes red colouration and death in clover?

There are many other factors that can cause sub-clover to turn red and die, these include;

• Moisture stress. A dry June-July can be a major contributor to the death of many sub-clover plants, particularly on the sandiest soils with the poorest water-holding capacity. Sub-clover is a relatively shallow-rooted species and often fails to persist on sandy soils following intermittent periods of moisture stress. This is likely to be exacerbated by poor root growth, resulting in moisture stress and an inability to extract nutrients.
• Waterlogging. Prolonged wet conditions can cause this condition particularly in combination with cold temperatures.
• Nutritional stress. Generally low phosphate levels and even boron deficiency can lead to red leaves and stunted plants. Nitrogen stress leads to red clover plants. Poor nodulation can be secondary to a variety of causes. Fertiliser application is not expected to alleviate symptoms in most cases particularly during dry conditions and winter.
• Cold stress.
• Soil acidity.
• Fungal root rotting organisms, such as rhizoctonia. A suite of fungal root rotting organisms can be present in older pastures reducing root growth and root hairs which are the site of nodule initiation. Without root hairs there is no nodulation. Fungal root rot stress mainly occurs in paddocks with continuous (3-4 years) sub-clover pasture, as organisms accumulate.
• Herbicide damage. Either within-season from herbicides that affect sub-clover or from residues, particularly from sulfonylurea herbicides, that have been carried over from the previous season.
• Sub-clover red leaf virus (SCRLV).

What is sub-clover red leaf virus and how can I test for it?

Symptoms of SCRLV on sub-clover include intense reddening of the older leaves starting at the leaf margins before extending across the whole leaf. However the symptoms vary depending on variety. Infected plants may be stunted and fail to set seed. While there are many causes of red leaves in sub-clover, only virus testing can confirm the presence of SCRLV.

SCRLV is a luteovirus which needs a constant green host for survival. It is not carried in seed. In irrigation or wet areas it persists in a range of perennial clovers including white clover, red clover and strawberry clover which when infected do not show any symptoms. The virus is spread persistently by aphids, which means that once an aphid becomes infectious it can spread the virus for the rest of its life.

Although SCRLV may be present in pastures it may only be part of a number of attributing factors which cause reddening of clover leaves and stunted growth. The virus alone is unlikely to cause plant deaths or lack of nodules on roots.

Fertiliser application (phosphorus, potash or nitrogen) does not alleviate virus symptoms.

Spraying aphids to prevent the spread of the virus is not recommend, especially at this time of the year as virus spread is most likely to have occurred much earlier in the season even though the symptoms are only becoming apparent now (late winter). Low numbers of aphids are often sufficient to vector the virus from nearby over-summering sources and these levels often go unnoticed during autumn.

Only virus testing can confirm the presence of SCRLV. Fresh clover plants can be submitted to the department’s Diagnostic Laboratory Services – Plant pathology services to be tested for SCRLV (and fungal disease) and confirm if it is present or not. Collect 10 red or dying sub-clover plants from different locations across the paddock, including their root symptoms. Shake off excess soil and place in a plastic bag. Fill out the lab form, if possible provide the name of the sub-clover sown, and mail to; DDLS Plant Pathology, DDLS - Specimen Reception C Block, Department of Primary Industries and Regional Development, 3 Baron-Hay Court, South Perth WA 6151.

For more information on SCRLV refer to Agriculture Victoria’s Subterranean clover: Virus diseases page.

How can farmers manage this season’s and future red sub-clover occurrences?

If testing reveals that SCRLV is not the cause of red leaf symptoms, the surviving sub-clover plants may recover and lose their redness with warmer spring weather, provided soil moisture remains adequate for plant growth. However, they will not be as productive as plants that have remained healthy and will most likely hay off earlier as soils start drying off.

There are several management options that will reduce the likelihood of future occurrences of sub-clover red leaf syndrome. These include;

• Crop older pasture paddocks to reduce the build-up of root rot diseases or sow grasses, such as annual ryegrass or forage oats, for winter and spring feed.
• Ensure sulfonylurea herbicides are not used in the year before a pasture phase.
• Sow an alternative legume, such as yellow or French serradella, which are well adapted to sandy soils on which sub-clovers often get stressed.
• If the clover pasture has not been inoculated recently with rhizobia, and poor nodulation is suspected, the new Group C strain can be introduced using granular inoculants or by sowing more clover.

For more information on managing sub-clover contact Phillip Nichols, Senior Research Officer, South Perth on +61 (0)8 9368 3547 or Paul Sanford, Senior Research Officer, Albany on +61 (0)8 9892 8475.

For more virus information contact Benjamin Congdon, Plant Virologist, South Perth on +61 (0)8 9368 3499.