Red Thread

Red Thread is a fungal attack and appears as pink patches of grass. If you look closely you will see the pink fungus on the leaf. The best way of checking this is to pluck some grass from the lawn and look at it close up. Fortunately, it doesn't kill the grass and the cure is simple, just treat with Soluble Iron in a strong dilution (2.5kg per 50 litres of water). Red Thread spores reside in the soil and wait for the right conditions to develop. 
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Fusarium, Dollar Spot and other fungal diseases

The photos show dollar spot and two fusarium attacks. These diseases can be controlled if caught at an early stage by applying Soluble Iron in a strong dilution (2.5kg per 50 litres of water). I had quite well developed fusarium last autumn which I treated with iron. It was a perfect cure and the grass fully recovered. My tip is to treat early and treat again if the problem returns. Iron is very low cost and avoids using a fungicide which is expensive and only available to buy if you have a pesticide certificate.
The fungus develops on the leaf surface before entering the plant through spores or damaged areas. Soluble iron effectively kills the fungus present on the leaf but does not provide long-term prevention. You can apply it as a spot treatment to target and eliminate the fungus in specific areas, but any untreated fungus on other parts of the lawn that hasn't yet infected the plant will survive. For this reason it is usually best to treat the whole lawn.
The STRI now recommend Iron for this purpose. Find Soluble Iron in my shop here SOLUBLE IRON
Fusarium - more info
Fusarium is a genus of fungi known for its resilience and widespread distribution. Its spores are ubiquitous in the environment, including soil, water, air, and on plant material. Here’s how they operate and their potential to infect and kill plants like grasses:
Fusarium Spores Environmental Presence:
Fusarium spores are microscopic and lightweight, allowing them to be carried by wind, water, insects, or animals. This contributes to their widespread distribution.
Dormancy
These spores can remain dormant for many years in soil or on plant debris, waiting for favourable conditions to germinate.
Favourable Conditions for Development
Fusarium spores germinate and infect plants when environmental conditions are conducive, typically including: Moisture: High humidity or wet soil promotes spore germination and fungal growth. Temperature: Many Fusarium species thrive in warm conditions (20-30°C). Plant Stress: Weak or stressed plants (due to drought, overwatering, or nutrient deficiencies) are more susceptible to infection. Host Availability: Grasses or other susceptible plants provide a food source for the fungus.
Infection and Impact on Grasses
Once the spores germinate Fusarium enters the plant through the leaf and invades plant roots, disrupting nutrient and water uptake. Many Fusarium species produce mycotoxins, which can kill plant cells and suppress their immune responses. Infected plants often show yellowing, wilting, and eventual death due to vascular blockage or toxin damage.
The use of Soluble Iron (Iron Sulphate)
Soluble iron releases iron ions (Fe²⁺), which can interact with oxygen in the environment to form reactive oxygen species (ROS). These compounds are toxic to fungal cells and disrupt their cellular membranes and internal processes. When dissolved in water, iron sulphate temporarily lowers the pH of its surroundings. Fusarium and other fungi struggle to survive in highly acidic conditions, making it an inhospitable environment.
Desiccation (Drying Out) Dehydration of Fungal Structures
Soluble Iron alters the osmotic balance on the fungal cell wall. The solution draws water out of the fungal spores and hyphae, causing them to shrivel and lose viability. It also creates a less favourable microenvironment for fungal growth on the leaf surface by reducing excess moisture, further inhibiting Fusarium's development. While the primary target is the Fusarium fungus, soluble iron can also toughen grass leaves by promoting chlorophyll production, improving their resistance to fungal colonization. 
Application and Effectiveness Mode of Application: Soluble Iron is dissolved in water and applied as a liquid spray for even coverage on grass leaves. It is effective when applied during the early stages of Fusarium development, especially in moist conditions where fungal spores are likely to germinate. Soluble Iron is more effective in damp conditions, where it can stay on the leaf surface long enough to act on Fusarium spores.
Residual Effects: Regular applications may be needed as soluble iron does not have prolonged residual activity. It primarily suppresses fungal activity and does not entirely eliminate Fusarium spores, which can remain dormant in the soil.