June 19, 2009

Rust has traditionally been a sporadic but potentially serious disease problem of sunflowers in Nebraska, particularly in irrigated production. Usually, the most damaging stage — indicated by reddish-brown pustules containing thousands of spores — develops in mid to late summer. However, this year abundant inoculum has already been identified in wild and volunteer sunflower in western Nebraska and could quickly spread to young seedlings.

Understanding the Sunflower Rust 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.

As temperatures cool in the fall to less than 50°F, spores (urediospores) are converted to dark, two-celled teliospores which serve as the overwintering stage of the fungus.

In early spring, teliospores germinate to produce basidiospores, which can infect sunflower seedlings. The basidiospore infections give rise sequentially to the pycnial and aecial spore stages. The aeciospores, formed in developing aecia, then re-infect sunflowers to create new urediospores, completing the life cycle. The life cycle of these early spore stages (pycnial and aecial) have been worked out in the lab under controlled conditions, but are rarely observed in nature.

2009 Early Season Occurrences

Over the last month, both the aecial and pycnial spore stages have been identified on volunteer and wild sunflowers from more than 30 fields planted with sunflowers in 2008 throughout the Nebraska Panhandle. Pycnia are found on the upper leaf surface and appear as circular, orange lesions surrounded by a yellow border (Figure 1). The flask-shaped pycnia then form aecia, which are found on the lower leaf surface directly below the pycnia . The aecia are recognized as clusters of small yellowish-orange cups filled with spores (aeciospores)(Figure 2).

These observations, although rare, are thought to be due to the unusually cool and wet conditions experienced in western Nebraska since early May 2009. Environmental conditions favoring infection and disease development of the urediospore stage include a minimum of 2-3 hours of leaf wetness and temperatures ranging from 55-85°F. New infections can occur every 7-14 days depending on temperatures.

These early sightings suggest that economically damaging epidemics are more likely to occur this year due to the early production of inoculum.

Last week we identified the uredial stage on volunteer sunflowers in Scotts Bluff County (Figure 3) — before any 2009 sunflowers had even been planted. The abundant levels of inoculum identified in many areas of western Nebraska could quickly move into newly emerged seedlings planted this year.

Managing Rust in Sunflower

In order to reduce the chance of severe, early infections:

• Destroy volunteer and wild sunflowers to break the disease cycle.
• Don't plant the 2009 sunflower crop near where the 2008 crop was planted.
• Use funigicides to limit damage and control the spread of the disease.

The greatest yield reduction potential occurs with younger plants, not only because of a greater degree of susceptibility, but early infection allows more time for multiple infections to occur during the season. New spores may still move in from another location later, but the longer the infection can be delayed after bloom, the less the damage to the sunflower crop.

It may be necessary to scout earlier than normal this year. Field monitoring should begin within several weeks after emergence. When deciding whether to treat, consider

• hybrid resistance to rust,
• whether anticipated weather conditions will be conducive to rust, and
• plant growth stage and potential threat to yield.

If rust pustules are on the upper two to three leaves nearest the head before pollination, consider treating to limit potential yield loss.

Fortunately, fungicides are very effective when application is timely. Several fungicides are registered for use on sunflower for rust, including Headline (pyraclostrobin), Quadris (azoxystrobin), and Folicur (tebuconazole).

Robert Harveson
Extension Plant Pathologist, Panhandle REC