According to the Missouri Botanical Garden, heat stress or fungal disease can lead to the yellowing of hosta. Scorch or heat stress leads to browning leaf margins and yellow areas between the main leaf veins. Fungal disease causes the base of the leaves to rot, which causes leaves to yellow.
Planting hosta in direct sunlight can result in scorch or sunburn, especially in USDA Plant Hardiness Zone 6. Southern blight is a fungal disease that causes rot at the base of the leaves, causing leaves to yellow, collapse and eventually die. An additional sign of southern blight is the development of a cotton-like webbing at the stem base and surrounding soil.