Tomato Ghost Spot is Caused By Botrytis cinerea. Symptoms Are Circular Discolouration On Green Fruit & Lighter Colouration On Ripe Fruit.
Causes of Ghost Spot on Tomatoes
The discoloured “halos” are caused when a botrytis spore germinates on the fruit but then dies due to unsuitable environmental conditions. Should it find suitable conditions then it could cause fruit rot with characteristic grey mould rotting of fruit caused by Botrytis cinerea. In my experience fruit rot is uncommon but a dead flower falling onto the calyx, lower down the truss, is sometimes the location of botrytis rot. The physical movement of the plant for pollination purposes prevents this to a large extent and is another reason I’m in favour of it. During humid (muggy) weather I often used a motorised high power mobile fan to physically move the plants for pollination purposes. I walked along the rows and the high pressure jet of air also blew any dying petals from the plants as I walked up and down the greenhouses.
Does Ghost Spot Make the Tomatoes Inedible?
It doesn’t affect flavour or make them inedible. Fruit with ghost spot are perfectly good to eat.
What it does do is decrease the marketability of fruit if we wish to sell it. So commercial growers try to ensure they don’t get it. Because if they do we, as consumers, tend to reject the fruit when its for sale.
Preventing Ghost Spot
As mentioned above the use of a high power fan removes the petals that can cause botrytis on the calyx. But it doesn’t stop tomato ghost spot.
For ghost spot to appear we need two things. The botrytis spore and sufficient humidity for it to germinate. The spore is in the air everywhere. As a student I conducted experiments where we exposed agar plates to the air for a few minutes. Botrytis was found in the majority of plates. Its in the air and widespread.
That means that thew condition isn’t dependant on rain or irrigation splash. Though it might contribute a little to the second factor.
For the spore to germinate it needs humidity. As I write this post the local humidity here has ranged from 74-96% RH in the last few hours.
Germination of botrytis spores, and infection of the host plant, is dependent on a film of moisture for 8 – 12 hours, relative humidity of at least 85%, and temperatures 55 – 75°F.
So a warm moderately humid day, or even a warm humid/misty night, is all that is needed to germinate the spores.
Outside it’s hard to control these conditions and ghost spotting is more common. But in a greenhouse it is much easier to control the humidity and keep it below the 85% needed. In tunnels it’s harder. They tend to retain humidity and ghost spot is more common. But getting the conditions right isn’t impossible and often depends on good ventilation.
The best botrytis control, beyond humidity control, is still good crop hygiene and growing practices. Strong health plants resist botrytis much better than weak plants.
Ghost Spotting and Botrytis Sprays?
There are no fungicides licensed in the UK for garden/amateur use. Some exist overseas for commercial use. There used to be some used by commercial growers in the UK but I’m not aware if they are still licensed for commercial use in the UK.
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