Fungal pathogens are a major pest of many different plant species. The fungi feed on the host plant, removing nutrients and weakening the plant. They damage plant organs such as the leaves or the roots so they do not function properly.

Plants have a range of chemical and physical defences to resist these fungal pathogens. Climate change, in particular increased CO₂, may change the relationship between pathogen and plant, tipping the balance in favour of one or the other. Elevated CO₂ may provide the plant with more resources to resist the parasite, or changes in the opening patterns of stomata may increase or decrease the opportunities for fungi to enter the leaves.

An example of a fungus 1: Rose Black spot (Diplocarpon rosae)

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Rose black spot is caused by a fungus, which infects roses. It infects the leaves, causing black or purple spots. The rest of the leaves often turn yellow and may drop off the plant. Having fewer leaves reduces the plant's ability to photosynthesise. It makes less food and so it grows less and produces fewer flowers. Rose black spot is not good news for the plant or the gardener.

How is it spread? Fungi produce spores called conidia when they reproduce. Black spot conidia move from rose to rose

in water splashes, when it rains or as they are watered
by insects or other animals (including people) moving from plant to plant.
as they are moved around during cultivation eg from a garden centre to your home.

The plant becomes infected when the conidia germinate on leaf tissue while the leaves are expanding. The leaves must be wet for at least seven hours for the conidia to germinate, and so for any infection, to occur. The fungus overwinters as mycelium in fallen autumn leaves. Black rose spot is treatable using fungicides and by removing and destroying infected leaves.

An example of a fungus 2: Sycamore tar spot

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As the name suggests, sycamore tar spot affects sycamore trees and other related species such as maples. The disease is caused by a fungus which infects the leaves causing large black spots on them. It doesn’t seem to cause the tree major harm, although it probably reduces photosynthesis in the affected leaves and if the leaves are badly infected they may fall a bit early in the autumn, which also reduces photosynthesis and so slightly reduces the growth of the tree.

How is it spread? The tar spot fungus grows and reproduces in the fallen sycamore leaves over winter. It releases spores in the early spring. The spores are very light but slightly sticky. They are carried up in air currents and stick to the newly emerging sycamore leaves. Once the spore is firmly stuck to a new leaf it germinates and the fungus penetrates the leaf. First an area turns yellow, and then it becomes a typical black ‘tar spot’.

How do fungi infect plants?

Watch this short (5 min) video to find out

How does elevated CO₂ affect tar spot infection of Sycamore trees?

Think about it:

You are going to look at some data on tar spot infections in the experimental forest.

How do you think carbon dioxide levels in the air around the trees will affect how much the leaves are infected by tar spot fungi?

Now have a look at how scientists have investigated this and what they have discovered – and see if you need to change your ideas!

Leaf traps collect the falling leaves in both ambient and elevated CO₂ rings. We can collect data on the tar spot infection rate by counting the number of spots on leaves for trees grown in both ambient and elevated CO_{2 .}If we observe a difference we can make hypothesis that we can test with further experiment

a) Record the number of tar spots on each leaf in each photo. Remember to record the photo number next to each count.

b) Calculate the mean number of tar spots for the control ring and also for the elevated ring (to find the mean in each case, divide the total number of tar spots you have counted by the total number of leaves you have observed).

c) Plot a histogram showing the two treatments

d) What difference, if any, do you observe in the amount of fungal infection between the CO₂ treatments?

e) Suggest two reasons for the effect of elevated CO₂on the rate of fungal infection you have observed

f) Post your graph and interpretation on the anonymous post it board at the bottom of this page

Ambient CO₂

Elevated CO₂

Results: Paste an image of your graph here and a line explaining how you might explain the data (this is anonymous)

How do fungi infect plants?

How does elevated CO2 affect tar spot infection of Sycamore trees?

Ambient CO2

Elevated CO2

How does elevated CO₂ affect tar spot infection of Sycamore trees?

Ambient CO₂

Elevated CO₂