[Howard]

Two of my courgettes picked up (or already had) some sort of disease - Dr Hessayon suggests it's almost certainly Cucumber Mosaic Virus. They were removed from the greenhouse and destroyed as soon as I realised I had a problem.

Now, two of my tomato plants are showing similar yellowing on their lowest leaves . Dr Hessayon suggests that it might be a magnesium deficiency (they are ready to be potted on into growbags from 5" polybags so will be pushing towards being potbound, I guess) but they've been grown in tomato growbag compost and I'm thinking this is unlikely.

It seems that Mosaic Virus is transmitted by greenfly and I've certainly not seen any yet and I've not 'captured' any on the sticky yellow bug catchers I have hanging in my greenhouse.

So, what are your thoughts? I shall be devastated if they've got the virus as they're the best tomatoes plants I've ever grown! 

[Lardman]

Could the courgette leaves have been damaged by sun scorch - magnified in water droplets ?

Id give the toms a dose of Andrews and a drop of feed to see if they pick up before doing anything drastic.

[Gardeningguru]

Howard, I would be rather cautious.

One has to remember that many other things can cause "spots" similar to those appearing on your leaves.  The leaves to me look otherwise healthy.

I would imagine that leaves and new growth would be curled and/or look dwarfed I believe they are other symptons of Mosaic virus.

As another poster said, water droples magnified by the sunlight can cause similar damage.  To me that doesn't look particularly worrying.

Others will know better, but I wouldn't recommend ripping everything out just yet.

[mumofstig]

The problem with toms deff looks like magnesium deficiencyand the RHS says:

Magnesium deficiency (right) causes yellowing between the leaf veins, often more noticeable on the lower leaves. It is most likely to occur in tomatoes grown on sandy or acid soils, or where high-potassium feeds have been over-applied. In severe cases premature defoliation can occur, but mild cases are unlikely to affect yield or quality. If deemed necessary, control with two foliar sprays of Epsom salts (54g per 4.5 litres water) at fortnightly intervals.

I wonder if the tomato grow bag had too much potassium for the young plants which probably only needed ordinary multi purpose until they were transplanted? Just an idea

[Howard]

Many thanks for the replies... perhaps it's not as bad as I first thought. 

I shall proceed on the basis that it's magnesium deficiency and deal with accordingly. Then wait and see!