Leptoxyphium, an Unusual Sooty Mold

Ornamental sweetpotato leaves with sooty mold at the petiole/blade junction

The sooty mold Leptoxyphium on the underside of an ornamental sweetpotato leaf

This blog is going to be a bit more technical than many of my posts, but I hope you'll find it interesting. A recent sample of ornamantal sweetpotato leaves from a greenhouse showed dark fungal growth at the top of the petiole and on the upper and lower surface of the leaf, just at the point of petiole attachment. The colonies could be scraped off easily, which is typical of sooty molds. The fungus was sporulating freely, with conidia (asexual spores) produced in drops of liquid at the tops of dark synnemata (tiny columns of fungal hyphae).

Synnemata of Leptoxyphium sp. on sweetpotato leaf

Top of a synnema of Leptoxyphium, at 400x

Using Seifert & Okada's key to synnematous hyphomycete genera in the 2011 book "The Genera of Hyphomycetes", the identification was made to the genus Leptoxyphium. The name means "slender sword" in English, possibly referring to the shape of the synnemata, but it is interesting that there are also awl-shaped (subulate) cells around the fringe of the spore-bearing area. This genus is also described on pp. 777-782 of Stanley Hughes's 1976 paper "Sooty Molds" (Mycologia 68(4): 693-820). Leptoxyphium species are a tropical to subtropical sooty molds, and rather unusual in that they often grow in association with glands and glandular trichomes of plants, rather than on insect honeydew. The good news for the greenhouse producer is that while it is an asethetic issue, this fungus is not going to harm the plants.

Geotrichum Sour Rot of Shrink Wrapped Sweetpotatoes

Recently, we received a sweetpotato sample in the clinic.  These sweetpotatoes were individually packaged in shrink wrap and ready to be microwaved up for dinner.   

There was only one problem… they looked like this and smelled even worse!

Geotrichum Soft Rot 

The Culprit??

Micrograph of Geotrichum candidum conidia

Geotrichum candidum causes a watery soft rot of sweetpotato.  Geotrichum soft rot has a distinctive fruit-alcohol-sour odor.  This disease requires conditions where oxygen is lacking and thrives at high temperatures.  This disease can begin in the field after flooding and can continue throughout postharvest storage in poorly ventilated areas.   

Signs of the pathogen include white tufts of mycelium on the outside of the roots.  The disease can be avoided by providing adequate drainage in the field and proper ventilation postharvest.  Roots should be packed dry because a thin film of water on the root surface can impede respiration and promote disease development. 

How is this a problem for shrink wrapped sweet potatoes??

Shrink- wrapped sweetpotato with white mycelium from Geotrichum candidum

The Sweetpotato Industry 

The majority of sweetpotatoes consumed in the US are purchased through retail fresh market outlets. Traditionally, sweetpotatoes have been marketed as individual roots stacked in bulk displays and priced per pound.  The times are changing and product convenience and ease of preparation are increasingly important to today’s consumers.  A new trend in shrink wrapping sweetpotatoes appeals to buyers that prefer microwaving to baking.  Individual roots are prewashed, dried, and heat sealed in plastic wrap.  Consumers can pop the packaged root in the microwave for 8 minutes and voilà, dinner on the table. 

Shrink-wrapping adds value to fresh market sweetpotatoes by enhancing the appearance, reducing weight loss, and allowing for individual labeling.  The downside to this is the potential for added postharvest spoilage.  Wrapping produce in plastic creates an ideal environment for microbes.  There is no way to completely remove surface microbes from sweetpotatoes and with the right environmental conditions you might have a sour rot problem on your hands.  Even though the roots are dried before packaging, fluctuations in storage temperatures can lead to condensation and moisture build up, which in turn creates a nice environment for pathogens like Geotrichum.  To avoid this problem, storage temperatures must be monitored closely.  


For more information on post harvest diseases of sweetpotatoes check out this link,
http://www.cals.ncsu.edu/plantpath/extension/commodities/sweetpotatoes_postharvest.pdf