Archive Article: 1998/11/07

7 November 1998

LATE October rains messed up plans to spray oilseed rape for phoma. Is this disease still a threat, and what should be done?

1 RECENT ADAS trials have highlighted the importance of well-timed sprays. For the most effective control of phoma, a fungicide must be applied as soon as the disease is found; typically this occurs in late October or early November. Delaying treatment if conditions are mild and damp can allow infection levels to increase very rapidly.

Fungicides are capable of giving control of light leaf spot and phoma leaf spot (and hence stem canker) when used as protectants. There are some differences in efficacy which become apparent under high disease pressure or when kick-back activity is needed. ADAS trials have shown that under high disease pressure, Plover (difenoconazole) and Punch C (carbendazim + flusilazole) have the edge on kick-back. But all the commercial fungicides are capable of controlling phoma leaf spot and light leaf spot when used as protectants. Downy mildew is also widely present and is causing extensive yellowing and early death of cotyledons in backward crops. On leaves, downy mildew is causing small bleached spots with a brown margin and these look similar to phoma spots. Considerable care is needed to avoid misidentification – phoma spots are usually green on the underside of the leaf.

Further rain is favouring spore release from rape stubbles, with symptoms appearing about three weeks later if the weather is mild. Crops that are close to rape stubbles are the obvious place to watch carefully for phoma development.

The threshold of 10-20% plants infected gives a good indication that an epidemic is under way and spraying is likely to be profitable.

John Garstang,

ADAS Boxworth, Cambridge

2 PHOMA is a very damaging disease of rape; symptoms on the leaves in autumn and winter will provide infection for stem lesions several months later in spring and summer. Lodging can occur with associated yield loss. Rape debris from previous crops will provide air-borne spores which infect newly-sown crops in the autumn.

Small crops are at high risk from severe phoma damage. Early infection could cause considerable leaf loss as well as later stem cankers.

Pressure was extremely high at the end of October, with visual symptoms becoming apparent and spraying required as soon as the rainy weather breaks. Growers should not delay if they get the opportunity because there is no way of knowing when the next spray time will be. Rainfastness will vary with the product – Plover (difenoconazole) is rainfast after two hours.

Downy mildew may cause problems on backward crops although forward crops will not need treatment. A small amount of nitrogen and an application of cleared formulations of mancozeb, such as Zebra Flo or Helm, will help if the disease is threatening establishment.

The rate of use is the same as for mancozeb on potatoes, with a cut-off for use at six leaves or at 30 December.

Downy mildew symptoms appear as a fluffy fungal growth bleaching the lower leaf surface of infected plants. Phoma affects the upper leaf surface as a bleached lesion with black spore cases as the lesions age.

Some varieties of oilseed rape will be more susceptible than others to phoma and light leaf spot. These are: Amber, Commanche, Falcon, Gazelle, Herald, Lightning, Lipton, Meteor, Pronto, and Synergy.

David Neale,

technical manager, HL Hutchinson, Wisbech, Cambridgeshire

3 PHOMA is rarely a problem in Scotland but if you do see symptoms on the leaves, Plover or Punch C would be the best fungicides to apply to protect against phoma and light leaf spot.

The latter disease is the most damaging disease of oilseed rape in Scotland and an autumn fungicide is essential for all varieties to stop the disease affecting yields. Although yield responses to fungicide were lower last season (250-350kg/ha) – a result of the mild winter and late appearance of light leaf spot – autumn fungicides still paid for themselves.

If this year is more typical, yield responses to light leaf spot fungicides can be much higher in susceptible varieties – 0.8-1t/ha.

Dr Simon Oxley,

Scottish Agricultural College, Edinburgh

WHO needs GPS on an Old Grey Fergie when Prince is on hand to ensure a straight furrow is ploughed.

The nine-year-old Jack Russell x Bedlington is a regular sight on Leicester contractor Phil Ashmores Ferguson T20 on the Midlands ploughing match circuit.

It all started when Phil put him on the bonnet as a puppy while repairing the machine. "He didnt mind when I started the engine and has been coming to ploughing matches with me ever since," says Phil. "He always likes to see where he is going, so he faces the front most of the time, but if I reverse he gets up and turns round."

Although he normally stays on the bonnet during competitions, Prince has disgraced himself on occasions. In one instance, he jumped off and chased a leveret, eventually catching and killing it. On that occasion, Phil had more than stubble to bury with his ploughing!

&#8226 Win a bottle of champagne by sending your caption for our photograph to Caption Contest, Crops, Quadrant House, The Quadrant, Sutton, Surrey SM2 5AS. Result in our 12 December issue.

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