Monday, July 30, 2012

Bacterial Fruit Blotch of Watermelon

Nothing says summertime like slicing open a nice juicy watermelon! North Carolina produced 7,200 acres of watermelons valued at just over $24 million dollars in 2010. Most watermelons are grown in the sandy soils of the Coastal Plains or in the northeastern region of the state. 

We’ve seen quite a few watermelon samples at the Plant Disease and Insect Clinic this summer. Typically the most common diseases on watermelon in NC are Fusarium wilt, anthracnose, Cercospora leaf spot, gummy stem blight, and downy mildew. This summer we have received several samples with greasy lesions on the surface of the fruit. The disease: Bacterial Fruit Blotch or BFB caused by Acidovorax avenae subsp. Citrulli, or AAC.
Bacterial Fruit Blotch: Note greasy lesion and cracking (Photo: E. Lookabaugh)
AAC causes disease on most cucurbits, including watermelon, cantaloupe, muskmelon, cucumber and pumpkin. Bacterial fruit blotch was first reported in the U.S. in 1965 but did not receive much attention until 1989, when major outbreaks occurred throughout watermelon production regions. The disease now occurs every year with varying severity. 
AAC is a seedborne pathogen. Seedlings from infected seeds generally do not perform well. Water-soaked oily areas can be observed on the underside of cotyledons or other young leaves. Lesions expand along the veins and, in severe cases, infected seedlings collapse and die. Sometimes symptoms are inconspicuous or absent and the infected seedlings get transplanted to the field. There the bacteria are spread to healthy leaves and developing fruit by workers, on contaminated equipment, or in rain and irrigation splash. 
Water soaking on the underside of cotyledons (Photo: APS image library)
Once deposited, AAC infects through the stomata (tiny pores). Infections are most likely when a film of water is present on leaves or fruit, so rain, irrigation, and high humidity favor disease increase. The window for fruit infection is relatively short and lasts from just before flowering to about three weeks later. Once wax deposition begins on older fruit, the bacteria can no longer infect. Typical fruit symptoms include greasy olive colored lesions on the surface of the fruit. Under humid conditions, the lesions quickly expand. The lesions do not extend below the rind so the fruit is generally not affected at first. Over time, the rind begins to crack, allowing for secondary fruit rotting pathogens to invade.
Later symptoms: Note inconspicuous spots on foliage in  B (Photo: APS image library)
The most important way to manage this disease is through the use of certified disease-free seed. Plants produced from infected seed may not exhibit symptoms if conditions are unfavorable for disease. This means that asymptomatic watermelons may produce infected seed that can spread the disease. Certified seed is produced and tested with special methods that minimize the risk of seed transmission.

Managing bacterial fruit blotch once it is found can be problematic. Always keep seedlots together when planting seeds directly into the field or greenhouse. Monitor plants and remove infected seedlings. If one seedlot is contaminated, avoid spreading disease to healthy plants. Use drip irrigation rather than overhead irrigation to limit splashing. Plant spacing increases air flow and reduces humidity. Seeds from rotten fruit left unharvested in the field can produce infected volunteer plants the following year, which serve as sources of inoculum. Remove all volunteer plants to prevent production of secondary inoculum! 

Preventative chemical sprays applied before and during flowering prevent fruit symptoms from developing. There is no complete form of resistance available in watermelons, but varieties vary in their susceptibility. Varieties with dark-green rinds tend to be less susceptible than varieties with light-green rinds. Also triploid seedless watermelons tend to be less susceptible than diploid seeded watermelons. 

So, remember always start your watermelons from certified, screened seed. Don’t save watermelon seeds for next year – use them for a spirited seed spitting contest instead!