Posted: 09/07/2012 at 14:22
I checked out the website that Bob suggested (Cladosporium Fulvum - leaf mould). The leaves on my tomato plants, especially the recently affected ones, don't show the same signs as those of Cladosporium Fulvum. Using a magnifying glass, they only have tiny brown dots on the top of the leaves and are totally clear underneath. In fact the more advanced leaves don't show anything underneath until the dots have formed into large clusters and the tops of the leaves start yellowing.
Re the ventilation, as mentioned previously, I have two atuomated window vents, as well as a ground level louvre window, and of course the door, the latter two of which are opened fully every day, and I can see the circulation wafting the leaves of the greenhouse plants.
The greenhouse is also planted in THE sunniest part of the garden, and has a good clear area around all sides.
I have contacted the helpline of another very large gardening organisation (I joined at GW Live), who, on being directed to the photo's on this website, stated immediately (and again, following consultation with a tomato expert) that the problem was not an infection, but is a deficiency in magnesium. They advised treating the plants with epsom salts every two weeks, until the problem clears.
A suggestion for the possible cause, was that I created my own potting mix (following an article in GW), and that the PH balance / nutrient level (and other things that I forget) is not perhaps what it should be, and in future, to use new ready made compost for any vulnerable plants like tomatoes.
I have since carried out a number of their suggestions to test for potential problems:
(1) Test the soils PH - in my case the soil test kit indicates a PH level of 6. The same kit suggests a PH range of 5.5 to 7.5 for tomatoes. I am therefore towards the acidic end of this range, but still within parameters.
(2) Check that the root growth is OK. My tomatoes are planted in purpose made plastic rings (the retail version of a large pot without a base), sited on the top of the larger growing pots. I checked the roots within the plastic rings and the larger pots. I was amazed. There is a massive, dense, healthy looking root system that goes down to at least 6" below the plastic rings (as far as I could get down into the pot), with no sign of water saturation.
Following all this advice (thank you all!), my plan of action is to follow Italophile's (a), (b) and (d) - the greenhouse is already in the sunniest spot, so not (c) - as well as his excellent watering advice, plus persevere with the epsom salts, and of course next year, not to use my own potting mix.
I will keep you up to date with the progress.
With all this talk of possible fungal infection, I forgot to ask, what do you do at the start of the season to limit the likelyhood of such an infection in the greenhouse?
Many thanks once again.