Vegetable News October 2005

During the dry weather the development of ringspot is slow and without conditions favourable for germination ie high humidity /leaf wetness, any spores released will not germinate. A site in Comber where Greenmount have been monitoring ringspot spores, has had low spore numbers counted over the last month.

A return to showery conditions will increase the risk so growers should regularly inspect crops for signs of disease.

White blister has developed in crops with high humidity and warm temperatures favouring development of the disease. Folio (sprouts, cauliflower, calabrese), and Fubol (cabbage) give effective control. Signum (sprouts, cabbage and cauliflower) is also moderately effective.

Powdery mildew is developing in crops and risk of continued development is high. Spread is by windborne spores and the disease is favoured by periods of high humidity and warm (15º-20ºC) conditions. High numbers of spores have been counted in the Comber spore trap.

Where mildew is severe the plants lose vigour. In Brussels sprouts the developing buttons can be affected leading to fine black spotting which can make the crop unmarketable. Where disease is not yet present azoxystrobin (Amistar) + chlorothalonil (e.g. Bravo) will give protection. Where disease has developed and provided crop is at an appropriate growth stage tebuconazole (e.g. Clayton Tebucon, Orius, Folicur) can be used to give control.

When using fungicides avoid any one type being used intensively by alternating products with active ingredients and from different fungicide types.

Note that Amistar (azoxystrobin) and Signum (containing pyraclostrobin) are both strobilurin type fungicides and Plover (difenoconazole) and Folicur (tebuconazole) are both conazole type fungicides.

Destroy all crop debris promptly to reduce disease inoculum.

The very high population of slugs earlier in the season will have been dramatically reduced by the extended spell of dry weather.

Desiccation of surface dwelling slugs will have occurred. Where it has been possible field slugs will have sought areas of shade such as field margins, amongst weeds or if already in the crop canopy will remain there. Soil dwelling species will move deeper into the soil

Cultivations will have added to the reduction in population.

However with more showery weather conditions, or where irrigation has taken place, the remaining slugs will become active and the most widespread, the field slug (Deroceras reticulatum), will breed again in September/October. By the autumn the populations will have built up again.

Slugs will be most active at night with peaks of activity at dusk and dawn. When slugs start their nocturnal foraging they move in a random search until they encounter food. They may find several different types of food and will stay and feed for longer on the most preferred food. Once they have learned the location of this food they will come back each night, returning by day to a nearby refuge.

When slug pellets have been applied across the soil surface the slugs usually encounter a pellet within the first hour of activity. If the pellet has not absorbed any moisture and is too hard the slug will abandon it and move on. Slugs feeding on metaldehyde, e.g. PBI Slug Pellets, become paralysed where they feed and cannot move to shelter. In dry, sunny or windy conditions, with little plant cover large numbers will desiccate and die. However if damp conditions prevail or if there is good plant cover the slugs do not dry out and can recover.

If feeding on methiocarb e.g. Draza they initially become hyperactive and move away from the feeding point before the poison immobilises them.

While slugs do move out of the field margins into a crop they do not usually travel more than 2-5 metres into the crop so most damage further within the crop is caused by slugs already in that area.

When soil is moist slugs will become more active again. It will be important to assess the risk of damage to susceptible crops. To do this use bait traps.

Traps consist of a cover e.g. tile or hardboard sheet about 25cm Square with a small quantity of bait. Various baits can be used – bran mixed with beer or chicken layers mash are both attractive to slugs. Two heaped teaspoonfuls of bait should be placed under each trap Only use traps when the soil is moist so that slug are sufficiently active. Pace traps across the field in a "W" formation with 9 - 10 traps per 20ha.The traps need only be left out for one night but need to be assessed early in the morning before the slugs leave the trap.

If an average of 1 or more slugs is found per trap then use of slug pellets is indicated.

Alternatively traps can be baited with metaldehyde slug pellets but this bait can poison other wildlife so is less environmentally friendly. If slug pellets are used then assessment should be over a 3 days period. It is worth treating if an average of 2 slugs/trap are found over the 3-day period.

Amistar now has full approval for use in leafy brassicas

Signum has full approval for use in carrots for sclertonia control. It will also give some control of alternaria

Tracer (spinsoad) has approval for control of caterpillars on cabbage, cauliflower and Brussels sprouts. The product is naturally derived from a fermentation process and was first discovered in the remains of a rum distillery in the Caribbean!

While reportedly giving equally good control as synthetic materials it is very safe with low toxicity to natural enemies such as ladybirds. It’s main mode of action is through ingestion but it also has contact and translaminar activity.

Ferramol slug pellets are a new slug killer with low toxicity to other organisms.

They contain ferric phosphate, a stomach poison that once ingested causes the slugs to stop feeding and death follows within 3-6 days. The pellets eventually degrade into naturally occurring plant nutrients. Unfortunately the price of this environmentally friendly product is more expensive than conventional slug pellets.

The following off label approvals have been issued since the last Vegetable News

Joan Hamilton

Edible Crops Development Adviser

Greenmount Campus, College of Agriculture, Food and Rural Enterprise

August 2005

Telephone (office) 028 9442 6683

Mobile number 077 7575 7251