Reintroducing Old Concepts for General Disease and Pest Management While Researching Ways to Biologically Control Cercospora Leaf Spot

Chemical control remains the primary avenue for managing plant diseases. In 1979, Sharvelle reported that $60 billion of the world's 1974 food supply was lost to diseases, weeds and insects. More recently, Wrather et al. (2001) estimated that in 1998 ten countries (producing 97.6% of the world's soybean crop) lost 28.5 M tons (U.S. $6.29 trillion) due to plant diseases. However, many plant pathogens have developed resistance to pesticides. In addition, some effective chemicals (e.g. methyl bromide) are being banned due to health and environmental concerns (Moffat, 2001). The need for effective pest control is critical to modern crop production; however, new methods must be developed that conform to current socio-ecological guiding principles. Long before the advent of chemical controls, plant diseases and other pests were managed by crop rotations, plant diversity, livestock use, and other cultural practices (Sharvelle, 1979). Much of this once common knowledge has been lost to growers because of the changed emphasis to crop monocultures and a heavy reliance on chemicals. However, current trends and environmental regulations indicate that research must re-examine, remodel and reintroduce some old concepts into modern alternative pest management strategies (minimal chemical use) that use crop diversity, cultural, improved water management and other biologically-based technologies to better manage diseases and other pests. 

Cercospora leaf spot (CLS) disease caused by the fungal organism Cercospora beticola Sacc., is one of the most important diseases of sugar beets (Beta vulgaris L.). The disease has been reported wherever sugar beets are grown (Bleiholder and Welzien. 1972) and results in significant losses of root biomass and sucrose content (Smith and Ruppel, 1973; Shane and Teng, 1992). Smith and Ruppel (1973) graded CLS disease severity on a scale ranging from one (low) to ten (high). Their findings indicated that a severity level as low as 3 could result in sugar losses of as much as 30%. Recent research by NPARL Agricultural Systems Research Unit (ASRU) scientists has shown that laccase enzymes, produced by some bacteria and tricoderma and basidiomycetes fungi, detoxify cercosporin (toxin produced by C. beticola) and allows the C. beticola pathogen to be attacked by various biological agents (Caesar-TonThat et al., unpublished, 2003.) This finding opens the door to several new biological approaches for the control of C. beticola and as well as many other plant pathogens. Research at other government and university laboratories such as USDA-ARS in Fargo, ND, and Montana State University-Bozeman, MT, have focused on identifying and understanding how to genetically introduce host-plant cercosporin resistance into sugar beets. However, no research is evaluating the use of cercosporin resistant antagonists for CLS control. 

Severe incidences of CLS disease often require multiple applications using different fungicides to prevent large economic loss even when sound crop rotation practices are used. The current lack of effective cultural control methods suggests that other, yet-to-be-identified secondary hosts (e.g., other crops and weeds) can sustain C. beticola populations for many years. Several plants (e.g., Beta spp. including red garden beet, Swiss Chard and mangel-wurzel) have been identified as secondary hosts of C. beticola. Several weeds are also known hosts of C. beticola including Chenopodium album L, Amaranthus retroflexus L, Malva rotundifolia L, Plantago major L, Arctium lappa L and Lactuca sativa L (Vestal, 1933). Other common weeds such as mallow, bindweed, winged pigweed, wild buckwheat, and the common unicorn flower have also been named as alternate hosts. ASRU scientists have recently identified safflower, a widely grown dryland crop in the Northern Great Plains, as an alternate host (Lartey et al., in Press). Clearly, these host plants can serve as an inoculum reservoir that maintains the organism through long periods, even when sugar beets are not planted. Therefore area wide weed control and cropping programs may play a major part of long-term CLS management strategies. 

Contributing Scientists: Robert T. Lartey (Plant Pathologist), TheCan Caesar-TonThat (Microbiologist) and AndrewLenssen (Weed Ecologist)