From Meg McGrath, Long Island Horticultural Research & Extension Center, mtm3@cornell.edu:
Downy mildew was confirmed affecting watermelon in the sentinel plot at LIHREC on 9/1. This is the third time this disease has been observed on this cucurbit type at LIHREC since monitoring occurrence of the different pathogen types started at LIHREC in 2008. Previous occurrences were in 2013 and 2015. The pathogen causing downy mildew in cucurbits exists as two major groups called clades. Within Clade 1 there is a pathogen sub-group (a distinct genetic cluster) able to infect watermelon. The other downy mildew pathogen members of Clade 1 infect squashes and pumpkin. These crops do not yet have symptoms in the sentinel plot. Downy mildew has been developing in the cucumber and cantaloupe plants in the sentinel plot at LIHREC. Clade 2 of the pathogen affects these crops.
Symptoms of downy mildew in watermelon are dark brown leaf spots of irregular shape.
Surprisingly there are no reports of downy mildew on watermelon logged at the Cucurbit Downy Mildew Forecast webpage (note there is a pull down filter above the map to select cucurbit crop type) that are near enough to Long Island to be an expected source of spores: nearest reported case is in South Carolina. Likely there is at least one unreported case of downy mildew on watermelon between the two reported occurrences. This is a reminder of the importance for growers and extension specialists to report occurrences, and also the importance to check crops for symptoms as well as being signed up to get alerts about occurrences from the Cucurbit Downy Mildew Forecast webpage.
While downy mildew does not infect fruit, leaves die prematurely when fungicides are not applied reducing fruit flavor because the fruit do not mature on a healthy vine and also resulting in fruit being exposed and thus at risk of sunscald.
Conventional fungicides. Pathogens causing downy mildew and Phytophthora blight are related; both are Oomycetes. Therefore they are sensitive to many of the same conventional targeted fungicides, and thus late season watermelon crops being treated for Phytophthora blight are also being protected against downy mildew. Targeted fungicides labeled for downy mildew and also labeled or expected to provide control of Phytophthora blight include Orondis Opti (FRAC 49+M05; Orondis Ultra is labeled for Phytophthora blight), Zing! (22; Gavel is labeled for Phytophthora blight), Ranman (21), Forum (40), Revus (40), Presidio (43), and Tanos (27). Omega (29) is also labeled for both diseases but has a 30-day PHI for melons. Previcur Flex (28) is only effective for downy mildew. Phosphorous acid fungicides are only effective for Phytophthora blight. It is important to alternate among fungicides in different FRAC groups for resistance management.
Organic fungicides. In addition to copper, there are several biopesticides approved for organic production: Aviv, Carb-O-Nator, Double Nickel, Ecoworks EC, GreenFurrow BacStop, GreenFurrow EF 400, LifeGard, MilStop, Oso, OxiDate 2, PerCarb, Procidic, Regalia, Romeo, Serenade ASO, Serenade Opti, Serifel, Sonata, Sporan EC2, Stargus, TerraNeem EC, Timorex Act, Triathion BA, Trilogy, and Zonix. LifeGard, Regalia, and products with a Bacillus species as an active ingredient act as plant activators and thus for best results need to be applied starting a few days before first infection, which is about a week before symptoms are first visible.
For more information about this disease and its management see the Cornell Vegetables website.