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!

Bacterial Leaf Spot: Acidovorax in the Greenhouse

It’s been a busy season for bacterial leaf spots on greenhouse ornamentals. Last week, we blogged about Xanthomonas leaf spot of English ivy. This week, we will take a look at leaf spots caused by Acidovorax sp.

Bacterial spot lesions on hydrangea (Photo: Mike Munster)

Over the past month or so, we have received two petunia samples, a verbena sample, and a geranium sample from 4 different greenhouses across the state. All of these samples had leaf or stem spots. The leaf spots had a water-soaked appearance and plants in advanced stages of infection were collapsing. After examining the spots under a microscope, we concluded a bacterium was the problem.

Water-soaked lesions (Photo: Mike Munster)

Stem lesions (Photo: Mike Munster)

Genera of bacteria that frequently cause leaf spot diseases in floricultural crops include Xanthomonas, Pseudomonas, and Acidovorax. Unlike many fungi, which can be identified by their unique spore types, bacteria are difficult to distinguish based on their appearance. Most bacteria look the same as they ooze out a leaf. It is important to know which species you have so you know whether or not the bacterium present is pathogenic or if it is a secondary saprophyte causing little damage. 

To identify the bacteria responsible for causing the leaf spots in these cases, we had to culture them and then perform additional assays. These assays identify bacteria based on their ability to metabolize different kinds of nutrients and compare the results with a database. Results identified the culprit as an Acidovorax sp. We then took this bacteria and inoculated a healthy leaf to make sure the same symptoms were produced.  

Inoculated leaf (Photo: Mike Munster)

Bacteria belonging to the genus Acidovorax have been isolated from geranium (Pelargonium hortorum) and petunia (Petunia hybrida) since the late 1980’s. The geranium and petunia strains appear very closely related to each other, but different from other strains of Acidovorax, possibly indicating they belong to a new, unidentified species of Acidovorax.

Management recommendations for Acidovorax are similar to those we discussed last week for Xanthomonas. Always start with disease free stock or cuttings. Plants with these symptoms should be removed and destroyed, including the planting mix. Healthy-looking plants adjacent to the diseased plants should be quarantined if possible. Acidovorax is found on petunias, verbena, geranium, hydrangea, and lantana. Greenhouse operators should be sure to inspect all plants in this group if Acidovorax is found.

Leaf spot symptoms on verbena (Photo: Mike Munster)

Leaf spot symptoms on petunia (Photo: Mike Munster)

Keep leaf/stem wetness periods to a minimum once plants come out of propagation, and avoid any kind of physical injury or wounding to the plants, which will aid entry by the bacteria. They can be spread from plant-to-plant by hand, insects, water splash, and even as aerosols. Avoid handling plants when wet, and frequently sanitize knives or other tools used on the plants.

Thoroughly clean and then sanitize pots, flats, surfaces, and tools with steam or chemical disinfectants. There are no effective chemical controls for application directly to the plants, but during propagation copper products can be used to hold back the bacteria somewhat.