However, insects have not been shown to be as important in the overall spread of black rot as are other methods. Black rot bacteria may also enter plants through natural plant openings above and below the soil surface. Above-ground openings in the plant include hydathodes present at the edge of leaves, and stomates distributed over the lower and upper leaf surfaces. Entry of bacteria through stomates may occur when plants have been subjected to heavy rains or irrigation. Another natural portal of entry is through a normal root system.
Higher plants develop prolific root systems by producing new roots that originate in older roots.
During this process, tissue-breaks develop in the older roots which offer portals of entry for bacteria. This portal of entry is most significant if the soil is saturated with water from rain, irrigation, or flooding. After black rot bacteria enter the plant, disease symptoms can occur near the original point of entry or throughout the plant because the bacterium moves systemically. After plants are infected, marginal leaf lesions can be found within 8 -12 days. It may take up to 43 days for symptoms to appear on leaves after infection has occurred.