Ramularia leaf spot & Beet mosaic virus
Ramularia leaf spot
Summary
Ramularia leaf spot (RLS) is caused by the fungus Ramularia beticola.
Identification
Symptoms are similar to Cercospora leaf spot. Older leaves are infected first. Light brown round to angular spots (4-7 mm) mature to dark brown, with silvery grey margins turning white upon sporulation. Lesions then become entirely white. Infected leaves turn yellow and die.
Importance
Occurs only sporadically.
Spread
Spores spread via wind and water splash. Mild temperatures and humid conditions favour spore germination.
Management
Weather affects RLS severity; warm, dry conditions help crops recover. Dense crops are more susceptible to damage, as are those deficient in sulphur. Fungicides are available.
Beet mosaic virus
Summary
(Photo: John Fletcher, Plant & Food Research Ltd.)
Beet mosaic virus (BtMV) is less common and causes less damage than BWYV, but can be more difficult to control.
Identification
Light green chlorotic spots appear on younger leaves first, in a circular pattern often with sharply defined margins. Chlorotic rings develop on older leaves with or without green centres. Infected leaves become stunted and distorted.
Importance
Low incidence: Up to 7% incidence of infected plants detected in infected crops in a 2016 survey in the North Island and up to 16% in the South Island. BtMV is aphid and mechanically transmitted. Mixed infections with BWYV can also occur.
Spread
Aphids (e.g. green peach aphid) are the primary source of infection. Feeding aphids acquire and transmit the virus very quickly (within minutes of starting feeding). However, aphids only retain the virus for a few hours after ceasing feeding. The virus can also be transmitted by sap between plants.
Management
BtMV has been recorded on several weed hosts such as fat hen, so good weed control is important for disease management. Over-lapping crops of beet and volunteers can also be important sources of infection. Spraying aphids to control disease transmission is not practicable. Insecticidal seed treatment may offer some early protection. No source of resistance is readily available.