* EU has found 35 contaminated shipments of black spot disease
* Row comes at sensitive time as EU seeks deeper trade ties
BRUSSELS, Nov 15 (Reuters) - The European Union is preparing a ban on South African citrus imports that could take orange juice off Europe's breakfast tables next summer and sour efforts in Brussels to broaden trade with Africa's biggest economy.
The move follows the interception of 35 citrus shipments this year from Europe's chief summer supplier that were contaminated with the fungal black spot disease, which growers in southern Europe fear could take hold in their citrus groves.
In response, the European Commission, the EU executive, is drawing up plans for a ban that could be adopted by European governments by the end of November, said a person with knowledge of the plan who spoke on condition of anonymity.
Another person close to the matter said EU trade chief Karel De Gucht told South African officials during a visit to Johannesburg this week that the contaminated shipments were "serious and problematic", and that a response was needed.
South Africa exports about 600,000 tonnes of citrus fruit - mainly oranges, lemons, limes and tangerines - to the EU each year worth some 1 billion euros ($1.3 billion). It is the main source of orange juice drunk by consumers in Britain, Germany and France during the European summer months.
The dispute comes at a sensitive time because the European Union is seeking South Africa's support to unlock stalled trade deals with sub-Saharan Africa.
During negotiations in South Africa this week, EU officials offered to improve the terms of a bilateral free-trade deal dating from 1999 by granting South African sugar farmers duty-free access to Europe for the first time.
For now the planned citrus ban would be largely symbolic, as it would only apply to this year's South African citrus harvest, which ended in October, meaning EU imports have already stopped.
But the bloc's food safety watchdog is analysing the situation and if its studies show the disease has a risk of taking hold in Europe's estimated 500,000 hectares of citrus groves, the EU could extend the ban.
The head of the South African Citrus Growers' Association, Justin Chadwick, warned the Commission against a ban.
"Global experts have confirmed that citrus black spot is not a risk, so a ban would seem unnecessary," he told Reuters.
While harmless to humans, citrus black spot causes unsightly lesions on the fruit and leaves of affected plants, reducing both harvest quality and quantity. There is no known cure, but fungicides can be used to control the spread of the disease.
It is found in many citrus growing regions in the southern hemisphere as well as China and the United States, but has never established itself in Europe.
Since 2011, the number of citrus shipments from South Africa found to contain black spot has averaged about 35 each year.
Following calls by citrus growers in top EU producer Spain to take a tougher stance on the issue, the Commission said it would be forced take action if more than 5 contaminated shipments were intercepted from the country this year.
"Every day, South Africa displays its inability to control the pests in its crop," Spanish farming association AVA-ASAJA said in a statement. "Political interests prevail over the risk that such imports pose to the future of Europe's citrus farms."
In its draft scientific opinion published in July, the European Food Safety Authority said the chance of citrus black spot taking in hold in Europe was "moderately likely". But it added there was a high level of uncertainty due to a lack of knowledge over how the disease would respond to the EU climate.
EFSA is due to finalise its assessment by the end of the year, and its findings will largely determine what further moves, if any, the Commission will take to restrict South African imports.
But a group of citrus black spot experts from countries affected by the disease said it had identified "factual errors and omissions" in EFSA's draft assessment, and that there was no recorded case of the disease ever having spread via fruit exports.
(Additional reporting by Olivia Kumwenda in Johannesburg; editing by David Evans)