I have 2 hosta that have leaves that have browned on investigation the whole stem easily pulls away. I wonder with the heat and humidity if its Southern Blight and what fungicide should I use.
Unfortunately, it does sound like your Hosta plants have a fungal infection. Crown Rot, also known as Southern Blight, can be caused by three different fungi from two main families of fungi. All three types produce mycelium – which is a mass of tiny, fine, white thread like filaments. If you look closely at the base of your hostas you should be able to see the mycelium to confirm this diagnosis.
Your description of the leaves easily pulling away from the stem is consistent with crown rot, where the base of the infected leaf becomes mushy. Hot, wet or humid summers provide perfect growing conditions for these fungi.
Crown rot is spread as the fungal spores are transported through the soil by water. Ensuring good soil drainage can prevent the fungi from settling near the surface of your soil where your plants grow. Transplanting infected plants, and not cleaning garden tools after working in an infected area also contribute to its spread.
Herbaceous plants and some woody plants are affected by crown rot, so it is recommended that infected plants be removed before the fungus spreads.
If the infection is minor or localized to your hostas, you can try: 1.removing dead leaves, 2. ensure that mulch isn’t in contact with the base (crown) of your hostas (as it holds moisture) and 3.improving the drainage of the soil around your hostas by adding compost to your plants. You can either purchase a bag of compost or add well rotted organic material from your home compost bin. Add compost several inches thick around each plant, avoiding the crown, radiating out about a foot to ensure the root ball can access the nutrients released by the compost. Check the soil around your hostas to see if the mycelium have spread beyond the two affected plants.
If your fungal infection is advanced you may notice mustard seed sized sclerotia (hardened fungal sacks made of mycelium and used as food storage) at the base of the hostas. They range from tiny white balls to orange/tan and can join to form a crust on the soil.
The fungicides required to control crown rot are not permitted for use in Toronto, so if you notice sclerotia at the base of your plants -or mycelium in the soil beyond your hostas, it is time to remove and discard your plants. Don’t replant hostas in the same place until you have removed the mycelium contaminated soil or allowed the soil to recover for several years.
To remove the fungal affected soil, dig a minimum of 6 inches beyond the diseased area, where the soil should have no obvious mycelium. Dig down to a depth of 8 inches. Replace the soil with a free draining soil blend.
Alternatively, if the site receives a minimum of two hours direct sunlight per day, you can solarize the contaminated soil. Sclerotia are killed at a temperature of 58 degrees celsius, so adequate sunlight is essential for solarization to be successful. I have attached a page that briefly describes the process of soil solarization.
Disinfect all tools used to handle the soil around your infected hostas to reduce the chance of fungal spores and mycelium fibres spreading to other areas of your garden.
Hostas enjoy regular water and nutrients so, if you are satisfied that they are planted in free draining soil, make sure they are watered regularly over the summer and have a layer of compost to feed them over the growing season.