This time of year, a vegetable grower’s attention turns to cool-season crops. Their hardiness and tolerance in the face of early frosts and dropping temperatures make them reliable extensions of the growing season before its inevitable conclusion in the winter. One of these crops is the almighty potato, a seasonal staple whose versatility lends itself to countless favorite holiday dishes (my sights are already set on the fluffy mashed potatoes that await me on Thanksgiving). However, growing potatoes is not an easy feat. A great number of plant diseases elect potatoes as their hosts, upon which they lay waste to once promising and bountiful harvests. Perhaps the most well-known of these diseases is late blight, Phytophthora infestans.
Phytophthora infestans is an oomycete – a group of organisms that share many characteristics with true fungi but are evolutionarily distinct from them. Oomycetes produce three kinds of spores – oospores, sporangia, and zoospores – that are each suited for different stages of the organism’s life cycle. Environmental conditions play a key role in determining the reproductive process and structures that make up Phytophthora infestans. In the Northeast, the disease overwinters on potato tubers, debris, and soil infected in previous years. Humid and wet weather causes sporangia located on this material to germinate and travel to nearby potato plants via wind currents. There, sporangia will directly infect them by way of a germ tube, a structure which allows the pathogen to break open and enter plant tissue. Under cool and wet conditions, sporangia will germinate and produce zoospores that swim along water-covered surfaces to infect their host.
In the early stages of late blight infection, potato leaf tips develop small lesions ranging in color from purple to dark brown or black. Lesions often have a chlorotic (yellow) border and can quickly expand to cover entire leaf surfaces. Following periods of humidity or rainfall, white sporulation may be visible on the underside of these lesions. As the disease progresses, entire plants can become defoliated as leaves shrivel and die. Late blight symptoms are also present on stems. Similar in color to leaf lesions, they may appear in any region of the stem and expand as disease worsens. Potato tubers may exhibit symptoms as well, turning abnormal shades of reddish-brown or purple and even producing white sporulation on outside surfaces. Through its infection, Phytophthora infestans detracts from their host plant’s vigor and makes them more susceptible to secondary infections of other fungal and bacterial pathogens.
The rapid progression of late blight, as well as the prolific dispersal and persistence of its reproductive structures, make it a particularly challenging disease to control. Sanitation practices, such as proper cleanup of spent material and sterilization of tools, are effective in minimizing disease spread. Crop rotation is also beneficial to reduce the risk of disease reoccurrence in a planting area. Potatoes suspected of having late blight may be submitted to the UConn Plant Diagnostic Lab for confirmation and management recommendations.
