I imagine that if Jamaicans were white they would be equally preferable to Canadian farmers. In Canada, the cost of living is expensive because it is necessary to stay warm in the winter. Jamaicans come in the warmest part of the year, and return before it gets cold. That is one reason why they are cheaper than locals. Of course, the locals would also need to stay warm in the winter, and there is really no seasonal agricultural work available in Canada in the winter.
I fail to see how this is racist. It is possible to live in Jamaica on what earns working for several months in Canada in the summer. It is probably not possible for local Canadians to do the same. It is purely voluntary: Jamaicans who do not like to pick fruit in Canada do not have to do so. If you think it is racism, that is simply your opinion, and I suggest that since (a) you are not Jamaican, (b) you have never had a career as a fruit-picker, (c) you have never worked in Canada, that whether or not this is "racist" is more a topic for Jamaicans who have picked fruit in Canada to comment upon than you.