P&PDL Picture of the Week for
April 11, 2005

Whitlow grass, the inconspicuous mustard

Glenn Nice, Weed Diagnostician, Department of Botany & Plant Pathology, Purdue University

Winter annuals don’t always get the attention that many of our summer annuals receive.  Almost everyone knows giant ragweed, velvetleaf, and common lambsquarters, but most people probably have not heard about Whitlow Grass, the inconspicuous mustard.

Whitlow grass, (Draba verna) not a grass at all, is also called shad flower, nailwort, and spring draba.  Although you may not have noticed this little mustard before, it can grow to form substantial mats in fields in the early spring.  It can be either an annual or biennial.  Small little stems, no taller than 5 inches, will grow erect from a rosette of leaves.  The leaves are no longer than one inch long.  Whitlow grass will bloom in February and May.  The stems will bear tiny white flowers 2/16 to 3/16 of an inch wide.  The flowers at first glance appear to have eight petals, but with closer inspection you will see that the four petals, characteristic of mustards, are only deeply lobed.  Like the flowers, the pods are also tiny.

Whitlow grass was introduced to North America, but has been around for some time.  An entry in the “Bulletin of the Torrey Botanical Club” in 1884 mentions Draba verna as being problematic in Pennsylvania gardens1.  Many of Whitlow grass’ brethren in the Draba genus prefer more extreme climates, being found in alpine ecosystems.  In 1897, Charles Schuchert and David White collected a Draba species from the arctic2.  Purdue University’s Kriebel Herbarium has several Draba spp. that were collected from Alaska.  I wonder if anybody is worried about controlling it that far North?

Although Whitlow grass is often found in our fields in the spring I have only had one question about it and that was concerning its identity.  Whitlow grass does not appear to be a control problem.  In many cases it dies before planting season, but where fall applied herbicides, early burndowns, cover crops, or tillage is used, Whitlow grass appears to be controlled effectively.

References:
1Thomas Meehan.  1884.  Survival of the Fittest.  Bulletin of the Torrey Botanical Club, Vol 11 No. 5, 53-55
2Theo Holm.  1900. Catalogue of Plants Collected by Messrs. Schuchert, Stein and White on the East Coast of Baffin’s Land and West Coast of Greenland.  Bulletin of the Torrey Botanical Club, Vol. 27, No. 2, 65-68.

 

Click on image to enlarge


Hollyhock Rust—NOT the same fungus as Soybean Rust!!!

Gail Ruhl, Senior Plant Disease Diagnostician, Interim P&PDL Director, Dept of Botany & Plant Pathology, Purdue University

Hollyhock rust, caused by the fungus Puccinia malvacearum, is the most common disease on hollyhock. The fungus survives the winter on infected leaf and stem debris from last year’s growing season. The fungus can also survive on the common, round-leaf mallow weed Malva rotundifolia throughout the winter. In the spring spores produced by the fungus are spread by splashing rain and air currents to newly emerging leaves.

Small yellow spots on the tops of hollyhock leaves will draw your attention to the presence of this fungal disease.  When you turn the leaves over, you will see characteristic signs of a rust infection, numerous orange-brown pustules.  All green parts of the plant are susceptible to infection. Rust infection will spread very fast from leaf to leaf, killing more mature leaves and making the plant unsightly.

Control: Remove and destroy the leaves on which rust is first observed. Fungicides, such as chlorothalonil (Daconil 2787), can be used to control the disease. In order to be effective, they must be applied at the first sign of disease and repeated every 7-10 days. Be sure to check the label for any precautions regarding use.

After flowering, the plants should be cut down and destroyed along with any leaves that have fallen. Removal of common mallow in the area will help insure that the fungus has no weed host material on which to survive the winter.

Images courtesy of Dan Egel, Purdue University

Early symptoms caused by Hollyhock Rust

Close-up of yellow spots on the top of the leaf

Close-up of young rust pustules on the underside of a hollyhock leaf. Spores develop in pustules and will spew forth when pustules erupt.

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service