|August 2005. This problem appeared in tomatoes about 6 to 9 months' ago. The grower in Solomon Islands shut down production to get rid of it, and then started again 2 months ago. The plants were doing well until last week. The plants are in polythene bags full of wood shavings. Watering/feeding is done twice a day.
Members suggested that there might be problems from whiteflies, aphids or green vegetable bugs. There was a possiblity of spider mites, which might be the cause of yellowing and drying of the leaves. If they are found, then a systemic insecticide should be given, being carefull to with hold treatments prior to harvest.
Leaf spots are noticeable and a number of fungi were suggested as a possible cause: Phytophthora, Cercospora, Alternaria and Fulvia fulva (Cladosporium fulvum or known by its 'perfect' name of Mycovellosiella fulva (a photo was given for comparison; lower, left). In the early stages of M. fulva, the leaves show yellow irregular shaped spots and, later, the underside of leaves show greyish mouldy growth and necrosis. A copper fungicide or Mancozeb will be effective against this and several other leaf spot diseases.
The other possibility was a nutrient deficincy because of the growing conditions, especially the use of wood shavings and liquid fertilizer. The Queensland University suggested that it is quite likely to be a problem in this medium, and being generously fed with an NPK-dominant fertilizer. Magnesium uptake is suppressed by high K levels. In soils, the Mg concentration is usually several times higher than K. If K exceeds Mg by a significant amount, there can be problems. The wood shavings would be starting with a pretty high K:Mg ratio, and the fertilization is likely to have gradually increased this. Urea or ammonia N sources might also acidify the potting mix, which would exacerbate the problem.
If it is that problem, it should be pretty easy to correct, and show a response quickly. Magnesium sulfate, sold as Epsom Salts, is generally available in the supermarket. It's highly soluble and can be applied to both soil and foliage. The soil application rate (for Epsom Salts, which is just under 10% Mg) would be about 50 g/m2, or 0.25 g/L of potting mix. For foliar spray, check the box. It should be applied to both potting mix and foliage. The leaves should show some greening within a few days, and no new symptoms appearing.
Note that calcium might also be low for the same reasons, and correcting Mg deficiency might allow a Ca deficiency to emerge. The Mg would also directly antagonize Ca uptake. So watch out for blossom-end rot of fruits, or shrivelled young leaves. If a problem is suspected, apply gypsum. This might be hard to get in the Solomon Islands, but it might be possible to get hold of some gyprock wall board and (e.g. builders' off-cuts) - soaking this would yield a suitable suspension of gypsum. In future, it might be advisable to include some dolomite (contains both Ca and Mg, but not very soluble) in the potting mix.