This article appears in the January 2018 issue of Potato Grower.
Zebra chip disease is a major problem for potato growers worldwide due to its ability to greatly diminish yield and tuber quality in afflicted plants. Zebra chip is caused by a bacterium named Candidatus Liberibacter solanacearum (Lso), which is spread between plants by the potato psyllid insect. Current best management practice is to prevent the spread of zebra chip by controlling the vector through the use of insecticides. However, potato psyllids are developing insecticide resistance, so control is becoming more challenging. Novel management options such as the development of resistant cultivars are therefore necessary to ensure long-term mitigation of zebra chip disease.
Major screening efforts have been conducted to find zebra chip resistance in existing potato breeding lines, which often contain wild potato relatives in their backgrounds. Out of hundreds of potato breeding lines, a handful were discovered that did not have observable zebra chip symptoms when infected despite becoming positively infected by Lso.
The major symptom of zebra chip disease is brown discoloration that occurs when potato tubers are freshly sliced or when fried. Food science has determined the reasons behind this symptom. The increased browning of freshly sliced tubers is due to the increase of plant-produced compounds called chlorogenic acids, the same compounds found in apples that result in brown discoloration after a bite is taken. This is called enzymatic browning because a plant enzyme protein converts the chlorogenic acid to a brown pigment when exposed to oxygen in the air.
Healthy tubers do not have much chlorogenic acid present, so fresh potato products like mashed potatoes stay white. However, zebra chip-afflicted potatoes have increased levels of this compound. As a result, potato products turn brown rapidly if the tuber is diseased.
The increased browning when frying tubers is caused by a different process called non-enzymatic browning. In this case, amino acids and sugars merge together when placed in the high temperatures of a fryer to form a brown-tinted product. This is called the Maillard reaction, and is best known for making toast brown. Indeed, over time, most fried potato products have some brown color develop as a result of this reaction. However, zebra chip-afflicted potato tubers have far greater levels of amino acids and sugars, so the brown discoloration occurs almost immediately upon hitting the fryer, leading to chips, fries and tater tots that look far overcooked although they were in oil for just a few seconds. In fact, such brown discoloration of fried chips often may appear as stripes, ergo the name of the disease, “zebra chip.”
What does this food science have to do with zebra chip tolerance? Well, as mentioned before, the discovered breeding lines do get infected with the causal pathogen, but do not develop symptoms; these tubers remain marketable. The answer of why lies within this food science. Tolerant potatoes do not seem to undergo changes in chlorogenic acids (which leads to fresh browning) or amino acids and sugars (which leads to fried browning) to the same extent as susceptible cultivars do. In fact, whereas susceptible varieties of potatoes can have hundredfold increases in these compounds when infected with the zebra chip bacteria, tolerant varieties have only about five-fold increases, far below any observable changes that could cause browning. So when susceptible varieties become unmarketable due to discoloration from zebra chip, afflicted tolerant varieties remain marketable.
Moving forward, various potato breeders are working to incorporate this observed zebra chip tolerance into potato selections and varieties to have good market qualities. This will hopefully ensure a future free of the negative aspects of zebra chip disease, even where it is present.