Introduction

Powdery mildew, which is caused by the fungus Leveillua rutae (syn. Oidiopsis haplophylli) on nasturtium (Tropaeolum majus L.), was found in southwest Florida for the first time in 2015 (Fayette et al. 2016). The affected plants were growing within a tunnel-greenhouse. The disease could become a limiting factor for the commercial greenhouse cultivation of this crop in Florida.

Nasturtium (Tropaeolum majus L.), also known as Indian cress, belongs to the Tropaeolaceae family and is a warm weather annual plant native to South America. Widely used as an ornamental, nasturtium is also increasingly being used as a medicinal and edible plant (Figure 1). All parts of the plant, including flowers, flower buds, and leaves, are edible and contain high concentrations of bioactive metabolites, which confer several medicinal and beneficial properties. Nasturtium is a rich source of vitamin C. The leaves have high concentrations of glucotropaeolin (a glucosinolate whose break-down products have anticancer properties), and the flowers are a valuable source of anthocyanins and carotenoids. (Garzon and Wrolstad 2009; Ghedira and Goetz 2013). In the United States, although the market of nasturtium is limited to local use as a specialty crop, it is highly valued and is increasingly distributed in specialty stores and farmer's markets as a nutraceutical food (dietary supplement).

Pathogen and Symptoms

On the foliage, symptoms of powdery mildew initially appear as yellow leaf spots that develop into angular and necrotic lesions (Figure 2). White fungal structures can be seen on both surfaces of the leaves, although there is usually a higher abundance on the abaxial (lower) leaf surface (Figure 3). In severe cases, the entire plant and flowers can be covered with the white spore-bearing structures of the fungus (Park et al. 2010). Plants with powdery mildew usually do not die, but become progressively weaker and unproductive.

The fungus causing this disease is an obligate parasite, meaning that it can only survive and reproduce on a living host. The fungus feeds on the host through specialized structures called haustoria that invade the host cells to obtain nutrients. The feeding process causes the cell to yellow and then die. The fungus reproduces through spores, called conidia, which are borne on upright conidiophores. Spread occurs when the conidia are splash dispersed by rain or irrigation water or when they travel longer distances by wind. Powdery mildew disease development is favored by moderate temperatures (65–80°F), high humidity, and low light conditions.

Management

Management practices that reduce plant shading and decrease humidity, such as increasing plant spacing and growing in well ventilated houses, might help with powdery mildew. Timing production to occur during cooler months, September to March when humidity is lower, may be helpful. Infected plants should be removed from the house to reduce the spread of inoculum within the house. Plants should be bagged in place to minimize spore movement rather than just carrying them out. Although no fungicides are specifically registered for use on nasturtiums, there are broadly-labeled biopesticides with potassium bicarbonate, biological controls, or oils as the active ingredient that might help to suppress powdery mildew on this host. Check product labels to be sure that application to nasturtium as a food crop is an option according to regulations in your state. Apply regularly starting before or at first symptoms.

References

Fayette, J., K.E. Hendricks, K.E, M. Ozores-Hampton, F. Di Gioia, and P.D. Roberts. 2016. "First report of powdery mildew caused by Oidiopsis haplophylli on garden nasturtium (Tropaeolum majus L.) in Florida." Plant Disease 100:646.

Garzon, G. A. and R.E. Wrolstad. 2009. "Major anthocyanins and antioxidant activity of Nasturtium flowers (Tropaeolum majus)." Food Chem. 114:44–49.

Ghedira, K., and Goetz, P. 2013. "Tropaeolum majus L. (Tropaeolaceae)." Phytothérapie 11:316–319.

Park, M. J., J.Y. Kim, J.G. Han, and H.D. Shin. 2010. "First Korean Report of Powdery Mildew on Tropaeolum majus." Plant Pathology J 26:204–204.