My Rice keeps getting diseased, how can I stop it?

Certain pests are treated in different ways. Stats such as immunity, pesticide, and herbicide will help slow the progress of these pests, but each one is dealt with in certain ways that are listed in scrolls that you'll find as you progress. Better quality seeds (that you get through mud or salt sorting in the winter/spring) are also more resistant to pests.

Catching snails and spiders will help you deal with bugs and pests. Midsummer drying will help kill weeds. Sometimes, certain rice diseases will progress but then disappear later in the growth cycle.

As you get more experience farming rice, you'll be able to tell the exact water level in your field. As in, you'll literally get a reading for the percent water level in your field once you've grown enough rice. A rule of thumb for water control is that you'll probably want to stay around 10-20% water level for the first three offshoots. Be careful in the third offshoot stage as the rice will get very thirsty and your field can dry up if you're not paying attention. By Summer 1/2, you'll want to do midsummer drying, which is draining your field of water until it's dry. While counterintuitive, this helps kill off weeds and prevents your rice from suffering overgrowth.

When the rice is in the Sprouting stage (which is usually Summer 2 or 3), you'll want to reintroduce water, upwards of 30% water level. Once your rice is ready to harvest, let the field drain and sit for a day before you actually harvest.