June 2004. PestNet is implementing a project in north Malaita, Solomon Islands. The aim is to test how farmers in an isolated area might use rural email to help with plant protection problems. In particular, how they might use PestNet.
Recently we held workshops on watermelon, sliperi kabis (Abelmoschus) and long beans, and did some follow up in villages where workshops were held on taro in January.
Watermelons at Takwa are being defoliated by a fungal disease, I suspect is gummy stem blight, although anthracnose is also a possibility. There is little vine or fruit infection. The varieties grown are Empire No. 2 and Flower Mountain.
It was suggested that the necrotic watermelon leaf spot looks somewhat similar to Cercospora leaf spot of watermelons in Darwin, Northern Territory, caused by Cercospora citrullina.
Didymella bryoniae (also known as Cercospora citrullina, the old name) is recorded from the Solomon Islands. There are no records of Colletotrichum orbiculare for the Solomon Islands. Colletotrichum gloeosporioides has been recorded from watermelon, but not in the Solomons. That is not to say though that they don't have these diseases.
It is easy to mistake "gummy stem blight/anthracnose" when on survey in different countries when all there is to go on are leaf spots (no fruit spots or vine symptoms). Leaves from watermelons with symptoms similar to the ones above have been tentatively diagnosed as "anthracnose" based on absence of stem symptoms (gummosis, stem canker and vine wilt) only to have the authenticated record come back as gummy stem blight (Didymella bryoniae). It seems the symptoms are sufficiently similar to be difficult to tell them apart in the field.
The important thing for farmers is "How do I control it?" The following information is well-known to everyone, but it is often the last thing that people do!
There are cultural practices that can help reduce levels of both diseases:
1) Seed borne aspect. Clean seed to start with is important.
2) One or two year rotations with non-cucurbit crops are useful in reducing the disease.
3) Destruction of the old vines after production is over (don't just leave them in the field).
There are also chemical control measures, but as usual, these are the more expensive options.