Rust, caused by the fungal pathogen Puccinia helianthi, is not a disease that we see every year. However, if it occurs early in the season, it can pose severe disease problems in sunflowers, particularly in irrigated fields.

Signs of rust observed this spring in volunteer sunflowers at the Nebraska Panhandle Research and Extension Center are a signal that conditions are conducive for disease to occur. Sunflower growers should destroy volunteer and wild sunflowers, be prepared to scout fields earlier than normal this year, and be ready to treat with fungicides to reduce the damage to crops.

The rust pathogen has a complex life cycle consisting of five distinct spore stages, all of which occur on sunflowers.

Likely the most familiar stage to consultants and farmers is the uredial. This stage is also the most damaging to the crop, and is characterized by reddish-brown, cinnamon-colored pustules (uredia) containing thousands of spores (urediospores). These "rust-colored" spores are the origin for the disease name and generally develop in mid- to late summer. This stage is also called the "repeating stage" because these spores can repeatedly infect new leaves and plants throughout the season. In the past, infections in Nebraska have often occurred late enough in the growing season that treatment has been unnecessary. 

Environmental conditions favoring infection and disease development by the urediospore (repeating) stage include a minimum of two to three hours of leaf wetness and temperatures ranging from 55 to 85 degrees F. Additional infections from these spores can occur every seven to 14 days if temperatures remain above 75 degrees for extended periods.

As temperatures cool in the fall (below 50 degrees), these spores are converted to dark, two-celled teliospores, which serve as the overwintering stage of the fungus.

RUST SYMPTOMS: From left going clockwise are pustules containing urediospores; teliospores; acia formed by pycnia on the lower leaf, recognized as clusters of small yellowish-orange cups filled with spores; and pycnia embedded in leaf tissue on the upper surface, appearing as circular, orange lesions surrounded by a yellow border.

In early spring, teliospores germinate to produce basidiospores, which then 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 uredia and urediospores, completing the life cycle.  Disease will not occur if the pathogen does not complete this cycle.

Since mid-May, the pycnial spore stages have been identified on volunteer sunflowers from research plots at the Panhandle REC planted to sunflowers in 2017. Pycnia are found embedded in leaf tissues on the upper surface and appear as circular, orange lesions surrounded by a yellow border. 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.

The appearance of the pycnial and aecial spore stages this early in the season is rare. However, similar observations were noted in late May 2009. Sunflower production throughout western Nebraska that year was severely damaged by high levels of rust.

The environmental conditions that spring were similar to the past several months — cool and wet. The sightings from 2018 are about two weeks earlier than in 2009, so the conditions are conducive for disease to occur.

This suggests that economically damaging epidemics are more likely to occur due to the early production of inoculum. To reduce chances of severe, early infections, it is important that volunteers and wild sunflowers be destroyed to break the disease cycle before the repeating (uredial) stage is formed, particularly if the 2018 crop is to be planted near sunflower fields from 2017.

The greatest yield reduction potential occurs when younger plants become infected. Early infections allow more time for secondary and tertiary infections to occur during the season, provided that wet and cooler weather persists. If infection can be delayed until after bloom, damage to the crop will be greatly lessened.

Producers in Nebraska are not accustomed to thinking about sunflower rust before August. However, it may be necessary this year to scout earlier than normal and be prepared to treat with fungicides to reduce the damage to crops. In 2009, disease from the uredial stage was observed on plants in early July, and fungicide applications were required.

In a situation like this, monitoring fields should begin several weeks after emergence. Using fungicides is an effective method for disease management provided they are not delivered too late. As a rule of thumb, uredial infections appearing on the upper two to three leaves closest to the head prior to petal drop will economically justify fungicide applications.

Field studies have determined that the optimal time for applying fungicides, if necessary, are at or before the early flowering stages (F4 to F5.5). A number of fungicides are registered for use on sunflower for rust control, including Aproach (picoxystrobin + cyproconazole), Priaxor (pyraclostrobin +fluxapyroxad) and Quadris (azoxystrobin), among others.

Harveston is a Nebraska Extension plant pathologist at the Panhandle Research and Extension Center. This report comes from Panhandle Perspectives, a regular column from the Panhandle REC in Scottsbluff.