Figure 1. Young pyncial lesions on sunflower volunteer cotyledons.	Figure 2. Advanced pyncial lesions on sunflower volunteer cotyledons.	Figure 3.  Young aecial lesions on sunflower volunteer.

 
May 26, 2010

With all the talk of stripe rust in wheat, don’t forget about a similar problem that may now be lurking in the weeds (literally). The cool wet weather that is so beneficial for stripe rust is also conducive to sunflower rust.

Disease Life Cycle

The rust pathogen of sunflower, Puccinia helianthi, has a complex life cycle consisting of five distinct spore stages — all of which occur on sunflower. In early spring, teliospores germinate to produce basidiopsores, which then infect sunflower seedlings. The basidiospore infections give rise sequentially to the pycnial (Figures 1 and 2) and aecial (Figures 3 and 4) spore stages. The aeciospores (formed in aecial pustules) then re-infect sunflowers, forming the more familiar rust-colored urediospores (Figure 5), and completing the life cycle.

The early stages (pycnial and aecial) were identified in volunteer sunflowers last weekend (May 22-24) in Scotts Bluff County. This indicates that the disease cycle has now been initiated, and that the inoculum required for development of economically damaging urediospores for later infection of commercial crops is present and waiting. A similar situation in 2009 resulted in earlier infections, often requiring earlier than normal treatment.

Management

In order to reduce the chance of severe, early infections, it is important that volunteer and wild sunflowers be destroyed to break the disease cycle, particularly near sunflower fields from last year. The greatest potential for yield reduction occurs with younger plants, not only because of a greater degree of susceptibility, but because early infection allows more time for multiple infections during the season.

It also may be necessary to scout earlier than normal and treat with fungicides to reduce the degree of crop damage. Scouting for rust symptoms, particularly in fields near 2009 sunflower crops, should begin within several weeks after emergence.

Fortunately, fungicides are an effective management method if applied in time. Several fungicides are registered for use on sunflower for rust, including Headline (pyraclostrobin), Quadris (azoxystrobin), and Folicur (tebuconazole).

Robert Harveson, Extension Plant Pathologist
Panhandle Research and Extension Center, Scottsbluff