To change a plants Sickness settings go to the plants blueprint so for example the BP_TomatoPlant and select the BP_PlantComponent in the Components panel, then in the Details panel and inside the Plant Settings Category find the Sickness category like the image below.

Sickness

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If Start Sick is true the plant will begin sick at the start of the game or when placed.

If Use Sickness is true the plant can become sick.

If Stop Growth When Sick is true the plant will stop growing when it is sick.

If When Sick Stop Health Regen is true the plant will not regen health while the plant is sick.

If Stop Vegetable Growth when Sick is true then the plant wont grow vegetables when sick.

Random Sickness

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If Random Sickness Chance is true the plant has a random chance to below sick, this is controlled by the Sickness Chance, the lower this value is the less chance the plant will become sick.

The Sickness Chance Time is how often in seconds that plant can potentially become sick using the Sickness Chance.

Sickness Damage Time controls how often the plant will take damage while sick, Sickness Damage controls the amount of damage the plant takes when sick.

Sickness Spread

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If Use Spread Sickness is true when the plant is sick it can have a chance of spreading its sickness to other plants.

Sicness Spread Distance is the max distance this plant can spread its sickness to other plants.

Sickness Spread Time is how often in seconds when the plant is sick it will potentially spread its sickness to other plants, depending on the Sickness Spread Chance, the lower this value is the less chance the plant will spread its sickness.