About 15 firefighters battled the blaze at around 9.05pm on Friday in Athelstaneford.
A grass verge next to the field was set alight with flames quickly engulfing the 30-40 metres squared crops.
Police were called in to investigate and are keen to trace a large group of youths aged between 16 and 19 seen in the area before the fire.
Constable Craig Purves from Haddington Police Station said: “While no one was injured as a result of this fire, the financial impact on the farm is likely to be devastating.
“We would urge anyone who can help us trace those responsible for this reckless act to contact police immediately.
“In addition, anyone with any further information relevant to this investigation should also get in touch.”
Those with information can contact Haddington Police Station via 101 and quote incident number 4512 of the 27th July. Alternatively, an anonymous report can be made to the charity Crimestoppers on 0800 555 111.
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