Septemer 2011. A field of watermelons from south Cameroon, near Yaounde with a number of complaints. The photos were uploaded on Flickr: www.flickr.com/photos/diagterrain_tc/sets/72157627552894606/. A similar poroblem(s) was seen in Senegal near Dakar.
Members remarked on a number of the conditions as follows:
a) leaf miners (IMG_3553)
b) yellow leaves followed by necrosis, probably a nutritional problem (IMG_3540-IMG_3547, 3556). The symptoms of chlorosis (yellowing) of the older leaves while the growing points remain green is typical of nitrogen deficiency where the N is translocated from older to younger parts of the plant. Note that the yellowing is concentrated at the leaf margin and in the areas between the main veins and the tissues there subsequently died. That pattern of lesion development is often related to more general nutrient deficiency; it is not typical of any kind of disease caused by a pathogen. In this case it is likely that nitrogen is the main problem, but other nutrients such as potassium are probably also in short supply. The amount of fern regrowth on the site and fern debris on the soil surface suggests that the area may have been cropped previously and allowed to revert to fern for some time. Under those circumstances it is likely that there has been a lot of nutrient leaching leading to the current problem. A soil analysis would be useful, and some idea from the farmer about the use of te land previously.
c) It was suggested that well-rotten manure should be placed around some of the plants to see this changes the collour of the leaves. (See, www.ehow.com/info_8418430_compost-helps-watermelon.html).
d) The yellowing of the leaves could also be due to poor root development, either because of insect or nematode attack. Check the roots, for galls or other kinds of damage.
In another email, the sender said that pumpkin and watermelons in nearby fields are also suffering from the same complaint.