Extension News

Ask the ISU Experts

Rhododendren killed by Phytophthora

Note to media editors: Got gardening questions? Call the Hortline at (515) 294-3108, Monday-Friday from 10 a.m. to noon and 1 to 4:30 p.m., or e-mail us at hortline@iastate.edu. For more gardening information, visit us at Yard and Garden Online, http://www.yardandgarden.extension.iastate.edu


I have some light green, apple-like objects on my flowering shrub.  What are they? 
The apple-like objects are likely the fruits of the flowering quince (Chaenomeles spp.). The flowering quince is an ornamental shrub grown for its attractive flowers in early spring.  Flowers are commonly orange, pink or red.  After flowering, plants occasionally produce a few apple-shaped fruits. The light green fruits are typically 2 to 2.5 inches in diameter. The fruits of the flowering quince mature in late summer/early fall. Mature fruits are edible.  While raw fruits are quite bitter, fruits that are cooked can be used for preserves and jellies. 

When can I cut back my asparagus foliage?
The asparagus foliage can be cut back to the ground after it has been destroyed by freezing temperatures in the fall. However, it is generally recommended that the dead foliage be allowed to stand over winter. The dead debris will catch and hold snow.  Snow cover helps protect the asparagus crowns from freeze damage.  Asparagus foliage allowed to remain in the garden over winter should be removed in late March or early April before the spears begin to emerge.

Why did my rhododendron wilt and die?
Rhododendrons and azaleas are especially susceptible to a fungal wilt disease called Phytophthora root rot.  The Phytophthora fungus enters the roots of the plant from infested soil and clogs the water-conducting vessels of the plant.  Affected plants may show poor growth and the leaves may roll up and droop from the branches, and eventually the plant may die.  Cutting into the base of the plant reveals reddish-brown discoloration below the bark. 

Wet conditions and poorly drained soil favor infection by the Phytophthora fungus.  Phytophthora root rot can be avoided by managing soil moisture and only planting rhododendrons in well-drained soil.  Infected plants should be removed completely, along with the soil around the roots, to remove as much of the fungus as possible.  Fungicides are often used in nursery settings to control this disease.

What causes brown spots on the leaves of my crucifers?
A bacterial disease of cruciferous vegetables (broccoli, cabbage, rutabaga, turnip) called bacterial leaf spot can cause small, water-soaked lesions to appear on the leaves.  As the lesions enlarge they become papery brown.

Another bacterial disease, called black rot, causes V-shaped brown lesions to appear on leaves. The veins in these lesions may appear black. Cabbages are especially susceptible to black rot.

Reducing leaf wetness can help to prevent spread of both diseases. Removing infected debris and rotating to non-cruciferous crops can help prevent survival of the bacteria from year to year. Copper-based fungicides can be sprayed preventatively.

Contacts :

Richard Jauron, Horticulture, (515) 294-1871, rjauron@iastate.edu

Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

A high-resolution version of the above photo suitable for printing is available through the following link: Rhododendren killed by a fungal wilt disease, 280 KB