Bacterial soft rot

Scientific name: Erwinia carotovora
Causal organisms: Bacteria

Host plants

Potato, sweet potato, cassava, onion, cabbage and other crucifers, carrot, tomato, beans, corn, cotton, coffee, banana, and many other succulent agricultural crops

Affected plant stages

All growth stages

Affected plant parts

Whole plant

Symptoms

On cabbage, an initial infection occurs on the outer petiole (leafstalk) which is in contact with the soil, and then progresses to its head. An infected head is watery and often has a complete head rot. The affected area becomes soft and mushy and generally turns dark in color. Soft rot infection on crucifers almost always emits a foul odor when other secondary organisms invade the infected tissues.

On carrot, an infected taproot has a soft and watery decay. The bacteria sometimes rapidly consume the entire taproot often leaving the epidermis (the peel) intact.

On cassava, an infested stem rots internally resulting in the wilting of young shoots or branches which is then followed by tip collapse and dieback.

On corn, an infected plant has a tan or dark-brown, water-soaked, soft stalk that suddenly collapses and is usually twisted. The initial sign is the premature withering and drying up of the tips of the uppermost leaves, then the lower leaves, soon followed by the appearance of a slimy soft rot at the base of the whorl . The decay spreads rapidly downward until the affected plant collapses. The diseased plant often has a foul odor.

On onion and garlic, an infected bulb is pale-brown and, soft and watery. The infected onion appears healthy on the outside and when cut open some of the inner scales are brown, wet, and have a cooked appearance. The neck of the infected bulb is soft when pressed and emits a foul smell. The onion maggot is an important vector in carrying the bacteria from one plant to another and causing wounds for the infection to enter the bulb. Bacterial soft rot often occurs during storage.

On pepper, the infected fruits collapse and hang like water-filled bags.

On potato, an infected tuber has cream to tan colored tissues that are very soft and watery. The diseased area often has a black border separating it from a healthy one. The soft rot decay is generally odorless but becomes foul and slimy when other secondary bacteria invade the infected tissues. Soft rot bacteria can sometimes consume the entire tuber, leaving only its peel in the soil.

On sweet potato, an infested tuber has watery and clear lesions on its tissues. The lesions rapidly enlarge in size, both diameter and depth. The infected tissue softens and becomes mushy or watery. Slimy masses of bacteria and cellular debris frequently ooze out from cracks in the tissues. During rapid development of the bacteria, the entire tuber may rot and collapse and sometimes leave only the outer skin intact. The decaying tissue may be cream or black in color and frequently gives off a foul odor which is caused by the secondary invading bacteria that grow on the decomposing tissues.

Conditions that favor development

1. Infected plant debris left rotten in the field.
2. Presence of onion maggot as the vector for soft rot in onion and garlic and root knot nematodes that cause lesions on the roots.
3. Plant wounds and injuries.
4. Hot and damp weather with plenty of rainfall trigger the disease to occur. Water is required for the bacteria to invade.

Preventive control

There is no known effective control measure of bacterial soft rot. The following practices can lessen its damage to the plant population;

1. For potato, plant the whole seed tubers
2. Proper land preparation to have a well-drained soil
3. Control nematodes and other insect pests that serve as vectors (carriers) of the bacteria to invade the plant tissues
4. If possible, avoid plant injury during weeding especially when the disease symptoms are observed
5. Remove infected plants immediately
6. Remove plant residues after harvest
7. Practice crop rotation by using crops that are not susceptible to the bacterial soft rot disease like soybean, forage legumes, and small grains

External links

• University of Kentucky. Corn stalk rots. http://www.ca.uky.edu/agc/pubs/ppa/ppa26/ppa26.htm
• WSU. Disease photo gallery. http://mtvernon.wsu.edu/path_team/diseasegallery.htm

References

• Brooklyn Botanic Garden. (2000): Natural disease control: A common-sense approach to plant first aid. Handbook # 164. Brooklyn Botanic Garden, Inc. 1000 Washington Avenue, Brooklyn, NY.
• Johnson, S. (1999): Blackleg and bacterial soft rot. Potato Facts. Bulleting No. 2493. University of Maine.
• Ploetz, R.; et. al. Editors. (1998): Compendium of tropical fruit diseases. APS Press, The American Phytopathological Society. Saint Paul, Minnesota, USA.
• Thurston, D. (1998): Tropical plant diseases. Second Edition. APS Press. The American Phytopathological Society. St. Paul, Minnesota, USA.