To solve this problem, we need to identify which part of the tobacco plant is infected by $Meloidogyne \, incognita. \newline$Let's analyze the options:$\newline \bullet$Option 1: Flower - Meloidogyne incognita is not known to infect the flowers of plants.$\newline \bullet$Option 2: Leaf - This nematode does not typically infect the leaves of plants.$\newline \bullet$Option 3: Stem - The stem is not the primary site of infection for Meloidogyne incognita.$\newline \bullet$Option 4: Root - Meloidogyne incognita, commonly known as the root-knot nematode, primarily infects the roots of plants, causing characteristic galls or knots.$\newline$Therefore, the correct answer is Option 4: Root.