The cause of this is usually that these people are allergic to ragweed. Allergy to ragweed is commonly referred to as "hay fever," and allergists tend to refer to it as "allergic rhinitis."
Allergic rhinitis can either follow a persistent pattern where it affects people several times per week, or it can be intermittent. Allergic rhinitis may also be affected by the pollens outside such as weeds -- in particular, ragweed.
Ragweed usually starts pollinating around the middle of August each year and is found all over the United States. Its pollens can travel far distances, being carried in the wind. This means that even though you may live in a city and don't see ragweed growing, that you can still be exposed to it and have symptoms. Ragweed season typically lasts until the first frost or around mid-October.
Besides the usual allergy symptoms, some people who are allergic to ragweed also develop an itchy mouth when eating certain foods during "ragweed season." The foods that people usually complain of are fresh fruits such as melons and bananas and vegetables such as cucumber, potato and zucchini.
Having an itchy mouth is not an uncommon complaint, and it is clinically called the Oral Allergy Syndrome (OAS). Basically, it is that some of the proteins in the ragweed are similar in structure to those found in the fruits and vegetables. So, in a strange way, it is almost as if your body thinks that you are eating the ragweed when you are eating fresh melon, for example.
Cooking these foods will cause the proteins to change, so that a person can then tolerate the foods. Although symptoms from the Oral Allergy Syndrome (such as mouth and throat itching, hives around the mouth and occasional swelling of the lips) are uncomfortable, they are rarely fatal. The best way to avoid the symptoms of OAS is to avoid the foods in the ragweed season, or to cook the vegetables.