**Explanation:**

Mycoplasma is a genus of bacteria that lack a cell wall and are therefore not susceptible to penicillin, which targets the cell wall. Instead, they have a unique plasma membrane that contains sterols, which provide stability and protection. This characteristic makes them resistant to many antibiotics that target the cell wall.

**a) They are pleomorphic:**
Pleomorphic means having the ability to change shape or form. Mycoplasma bacteria are pleomorphic, meaning they can adopt different shapes and sizes. They can range from spherical or ovoid to filamentous or irregular shapes.

**b) They are sensitive to penicillin:**
This statement is incorrect. Mycoplasma bacteria are not sensitive to penicillin because they lack a cell wall, which is the target of penicillin. Antibiotics like penicillin inhibit the synthesis of the bacterial cell wall, but since mycoplasma do not have a cell wall, they are not susceptible to the effects of penicillin.

**c) They cause diseases in plants:**
Mycoplasma can indeed cause diseases in plants. They are responsible for various plant diseases, including aster yellows, citrus stubborn disease, and corn stunt disease. These diseases can lead to stunted growth, yellowing of leaves, and overall reduced plant health.

**d) They are also called PPLO:**
PPLO stands for Pleuropneumonia-like organisms, which was a term used to describe mycoplasma before it was classified as a separate genus. Mycoplasma was initially classified as PPLO due to its resemblance to the bacteria that cause pleuropneumonia in pigs. However, later research revealed that mycoplasma is a distinct genus on its own.

In conclusion, the statement that is incorrect about mycoplasma is option B: They are sensitive to penicillin. Mycoplasma bacteria lack a cell wall and are therefore not susceptible to penicillin.