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First Report of the Early Spore Stages of Sunflower Rust, Caused by Puccinia helianthi, in Nebraska
Robert M. Harveson, Panhandle Research and Extension Center, University of Nebraska-Lincoln, Scottsbluff, NE 69361
Corresponding author: Robert M. Harveson. firstname.lastname@example.org
Harveson, R. M. 2010. First report of the early spore stages of sunflower rust, caused by Puccinia helianthi, in Nebraska. Online. Plant Health Progress doi:10.1094/PHP-2010-0315-01-BR.
Sunflower rust, caused by the macrocyclic and autoecious fungus Puccinia helianthi, has traditionally been a sporadic but potentially serious disease problem of sunflowers in Nebraska, particularly in irrigated production. Historically, infections have often occurred late enough in the growing season that fungicide treatment has been unnecessary.
Pathogen Life Cycle
The pathogen has a life cycle consisting of five distinct spore stages, the most damaging and commonly observed spore stage being the uredial stage. The uredia consist of reddish-brown, cinnamon-colored pustules containing thousands of spores (urediospores). This stage of the cycle is also called the "repeating stage" because it can repeatedly infect new leaves and plants throughout the season as long as conducive weather conditions continue. As temperatures cool in the fall (<10°C), these spores are converted to dark, two-celled teliospores which serve as the overwintering telial stage of the fungus (1).
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. Flask-shaped pycnia can be found embedded in leaf tissues on the upper surface (Fig. 1). Aecia, which develop directly from the pycnia, are usually found on the lower leaf surface directly below the pycnia (Fig. 1). The aeciospores, formed in developing aecia, then re-infect sunflowers to create new uredia, completing the life cycle. These early spore stages (pycnial and aecial) of P. helianthi have rarely been observed, and thus much of their biology and conditions for development have not been investigated (1).
Survey of 2008 Sunflower Fields
The spring of 2009 in western Nebraska’s Panhandle was characterized by unusually cool conditions with above average precipitation. Temperatures during April, May, and early June were 13°C cooler with 12 cm higher rainfall than the 30 year average for this area. On May 23, symptoms suggestive of the pycnial stages of P. helianthi were first observed on cotyledons of volunteer sunflowers in a home garden in Gering, NE. Pycnial lesions were circular and orange measuring 2 to 5 mm in diameter and surrounded by a yellow halo (Fig. 2). Within three days, the aecial stage was observed on plants in this same location, and subsequently both spore stages were additionally found occurring in multiple locations throughout Scotts Bluff Co. Aecia were recognized as clusters of small yellowish-orange cups filled with spores (Fig. 3).
Because of the continued cool and wet weather throughout May, a survey of production fields in western Nebraska was conducted over the next four weeks (26 May to 19 June) to determine the incidence and distribution of these early spore stages of P. helianthi. Perimeters of fields planted to sunflowers in 2008 were scouted for diseased plants, aimed primarily at cultivated sunflower volunteers from the previous year. The survey included more than 50 locations monitored in nine counties throughout the Panhandle, and also included various wild sunflower species found in ditchbanks and disturbed areas in close proximity to fields.
After the initial sighting in the home garden, more than 150 samples were collected and examined in the lab up to mid-June. The pathogen’s early spore stages (pycnial and/or aecial) were identified from 44 (85%) of surveyed fields/sites representing eight of the nine counties surveyed, including Scotts Bluff, Morrill, Box Butte, Sioux, Cheyenne, Kimball, Banner, and Duel. No rust was identified during the survey period from Sheridan Co., located in the northernmost sunflower-growing area in Nebraska.
The pathogen was readily identified from both cultivated volunteer plants and wild sunflower species in 2009. The widespread presence of the early stages of rust throughout the sunflower production areas of Nebraska resulted in much earlier uredial infections than normal. The uredial stage was first identified in Scotts Bluff County on June 8 - before any commercial sunflower fields had been planted. The first infected commercial fields were found in late June. Thus the major impact from these early infections was the need to treat new crop sunflowers with fungicides for rust by mid-July. Growers in Nebraska are not accustomed to spraying for rust until mid-August, if at all. For those fields with early infections that were not treated, substantial losses are anticipated since previous work under greenhouse conditions determined that plants infected prior to bloom experienced up to 40% yield losses (2). Recent field studies additionally observed losses greater than 80% from untreated infections compared with those treated with fungicides (3).
This study additionally documented multiple spore flushes from sites that were visited more than once. This was determined by observing volunteer or wild sunflower plants containing pycnial lesions present on young leaves with aecia on older leaves of the same plant; or by noting aecia on leaves additionally infected with old uredia (Fig. 4). The first finding of the pycnial and aecial spore stages occurred the last week of May, and only plants in the 4 to 6 leaf stage were infected at that time. This suggests that spore release and infection for that first event likely occurred previously in early to mid-May (2 to 3 weeks earlier). The second round of infections was noticed from many of the same locations the second week of June, which also coincided with the first uredial infection sightings. Lastly, a third flush was documented from three locations during the last week of the survey.
A recent report found the aecial stage of P. helianthi to be widespread throughout production fields in North Dakota and Minnesota during 2008 (3). However to my knowledge, this is the first report describing the occurrence of either of the early spore stages in Nebraska, and the first documentation of the natural occurrence of the pycnial stage in sunflower production. It is not known whether the high incidence of the early spore stages of rust is a common occurrence or simply a consequence of the cool, wet environment experienced during 2009 in Nebraska.
The knowledge obtained from this study may also build a foundation for developing a novel method for disease management. Since it is known that the pathogen survives over the winter as telia (1), it must first progress through the pycnial and aecial stages before initiating the economically damaging uredial stage. Therefore, if one knows where, approximately when, and what to look for in recognizing these early spore stages, their appearance may eventually serve as a simple predictive tool for determining the potential for later rust epidemics and estimating the need for making fungicide applications in commercial sunflower crops assuming weather conditions favor infection and disease development by the fungus.
1. Gulya, T., Rashid, K. Y., and Masirevic, S. M. 1997. Sunflower diseases. Pages 263-379 in: Sunflower Technology and Production. A. A. Schneiter, ed. ASA, Madison, WI.
2. Gulya, T. J., Venette, T. R., Vennete, J. R.. and Lamey, H. A. 1990. Sunflower rust. Ext. Serv, Bull. PP-998. North Dakota State Univ., Fargo, ND.
3. Markell, S., Gulya, T., McKay, K., Hutter, M., Hollingnsworth, C., Ulstad, V., Koch, R., and Knudsvig, A. 2009. Widespread occurrence of the aecial stage of sunflower rust caused by Puccinia helianthi in North Dakota and Minnesota in 2008. Plant Dis. 93:668.