Sour rot on tomatoes

Sour rot is a common decay of tomatoes everywhere they are cultivated, especially when temperatures are above 30°C. The decay can start in cracks, cuts, or insect bites in the skin of the fruit or at the brim of the scar of the stem. The disease develops rapidly, and the affected tissues become soft and watery. The skin in the affected area cracks, and a white, gaseous scum with a sour smell forms. At this stage, there is a proliferation of fruit flies, Drosophila melanogaster, which distribute spores and fragments of mycelium to other fruit with lesions of the skin. Sour rot can be diminished with careful management from the field to the shelf.

Sour rot is caused by Geotrichum candidum, a common soil-borne fungus that causes the disease not only in tomatoes, but also in citrus fruits and vegetables. Also, it can be a contaminant on tomato-handling equipment.

Symptoms

Numerous fungi cause fruit rots on tomato. Lesions are water-soaked, and may have a white scummy growth in the cracks. Lesions often occur near the stem end scar. The sour-rot pathogen is a type of yeast. Fruit lesions may initially be watery, but later become coated with fungal growth. This growth on infected fruit resembles a thick, gelatinous mass similar in appearance to cottage cheese and remains relatively firm unless a secondary infection by a soft rot bacterium occurs. The odor of these lesions is distinctive and is similar to that produced by lactic acid bacteria, hence the disease name, sour rot.

About and damage

G. candidum, like other fruit-rotting organisms, is an opportunistic pathogen that can live on plant debris in the soil. Because it is a saprophyte, it is ubiquitous in the natural environment and can be found in almost all soils. The sour rot organism may be windborne or splashborne on soil particles to surfaces of fruit in the plant canopy. Tomatoes are most resistant when they are relatively dry and firm. Mature green tomatoes are more resistant than are ripe fruit.

Mechanical injuries that occur during harvest and handling are a frequent site for decay development beginning on the fruit surface. Chilling injury and water congestion can compromise a tomato fruit’s natural resistance to decay and allow sour rot to develop and produce a watery decay.
Periods of persistent rainfall, fogs following rainfall, cool mornings, or exposure to chilling temperatures in the field can increase the incidence of sour rot.

Harvest and transport of wet fruit also can result in an increased incidence of disease. The potential for development of sour rot and other decays after harvest will be lowest if the plants are dry and free of decay at the time of harvest. When harvested and placed in bins, infections can spread to adjacent and nearby fruit, leading to a nest of infection.

Control

• Fruit should be kept off the soil (if possible), and surface wetting should be minimized. 
• Furrow or drip irrigation is preferred over sprinkler irrigation.
• Preventative fungicide sprays may be required if a “zero tolerance” for defects production system is needed.