Fungal pathogens have the potential to bring about serious reductions in the quality and quantity of lettuce produced  in Arizona.  Examples include Bremia lactucae, the pathogen that causes downy mildew; Erysiphe cichoracearum, the causal agent of powdery mildew; and Sclerotinia minor and S. sclerotiorum, the pathogens responsible for leaf drop. 

Fortunately, each of these fungal diseases does not cause major economic losses during every growing season.  However, given favorable environmental conditions for disease development, one or more of these diseases can develop rapidly and lead to extensive crop losses.  Depending upon the price of lettuce at the time of the crop loss, economic losses can be severe.  Without question, successful management of these fungal pathogens is essential for realization of improved quality and maximum yield potential of lettuce in Arizona.

Current management strategies for, downy mildew, powdery mildew, and Sclerotinia leaf drop are heavily reliant upon agrochemicals.  Because of increasing environmental concerns as well as potential development of tolerance to fungicides by fungal pathogens, there is no guarantee that materials currently in use will be available in the future.  New and more effective, as well as environmentally compatible methods of disease management are needed.  New disease management chemistries currently are being developed by several different agrochemical companies.  The novel modes of action of these compounds offer the possibility of controlling the above mentioned pathogens in new and exciting ways.  One new class of fungicides, called strobilurins, is derived from a fungicide produced naturally by some wood-rotting mushroom fungi.  The strobilurins have a broad spectrum of activity on plant pathogenic fungi, are very low in mammalian toxicity, and have been designated “reduced risk pesticides” by EPA.  Another unique product under development does nothing to fungi directly; however, when applied to plants this chemical induces systemic resistance in the plant against certain plant diseases.  This material, called Actigard,  induces plant resistance at very low use rates and also has been designated a “reduced risk pesticide” by EPA.  Also, interest in “organic” production requires evaluation of disease control products that will meet these requirements.  Finally, cultivars of lettuce may vary in their genetic susceptibility to some of these lettuce diseases, offering the possibility of cultivar selection as a disease management tool.

Comprehensive field trials were conducted last year to test the ability of some new compounds to control powdery mildew and Sclerotinia leaf drop of lettuce.  Additionally, several cultivars of lettuce were evaluated for their relative susceptibility to powdery mildew.