Wednesday, April 18, 2012

Plug Up Your Nose! Bacterial Soft Rot Arrives in the Clinic

There is nothing worse for clinic diagnosticians than receiving a package that smells before it’s even opened. If we are really unlucky, the smell is the pungent aroma of a bacterial soft rot sample. Last week, we were unfortunate enough to open one of these smelly packages to find a slimy mess of a tomato plant with a bad case of soft rot.
Bacterial Soft Rot on Tomato (Photo E.C. Lookabaugh)
Bacterial soft rot is easy to diagnose from the soft rot symptoms, and the unique, fishy aroma. It is caused by Pectobacterium carotovorum subsp. carotovorum (formerly known as Erwinia carotovora subsp. carotovora). These bacteria have a very wide host range and can also be a significant problem on potato and cabbage. Soft rot bacteria are ubiquitous and are found in relatively low numbers throughout most greenhouse production systems. 

Soft rotting bacteria are often called “opportunistic” pathogens because they are not able to directly penetrate host tissue – they need free water and a wound or other opening for infection to occur. Mechanical injuries, damage caused by insect feeding, or previous infections by pathogens like Phytophthora and Pythium allow soft rotting bacteria to enter the plant. Once introduced into an infection court, these bacteria quickly multiply to very high numbers. Soon after infection, a water-soaked lesion can be seen around the point of entry. As the bacteria multiply, they produce enzymes that quickly break down succulent host tissue. In the case of tomatoes, the soft inner tissue (pith) of the stem breaks down when infected. When the pith is lost, the stem becomes hollow and collapses. Soft rot can progress very rapidly under warm, wet conditions. Within 48 hours, soft rotting bacteria can turn a plant to mush.

The major means of controlling this disease is sanitation. The bacteria do not move through air but can spread mechanically on dirty equipment or on workers’ hands. Infected plants should be removed and destroyed. Mechanical equipment and pruning tools should be properly sanitized with a bleach solution. Workers should wash their hands thoroughly before working and after touching diseased plants. Avoid pruning, transplanting, or harvesting in wet weather. This disease is usually not important in field-grown tomatoes, but can become problematic after heavy rainfall or damage from hail storms. Avoid injuring fruit during harvest. Water used for washing tomatoes in packing houses should be treated to kill bacteria. However, washing infected tomatoes will not stop the disease from progressing; it will only serve to contaminate the water source. Avoid planting tomato in rotation with other host species.
Soft Rot on Cabbage (Photo: PDIC Database)