Joe Nunez
UCCE Farm Advisor, Kern County
Vegetable Crops - Plant Pathology
July 29, 1999
SUMMER PLANTING OF POTATOES
Summer planting of potatoes is arguably the most difficult crop to grow in the Southern San Joaquin Valley. The most challenging aspect of growing potatoes during the summer heat is getting a plant stand. It is not uncommon to plant a field two or three times before getting a satisfactory plant stand during the summer planting period. Establishing a plant stand under high temperature conditions can be difficult, but steps can be taken to improve the chances of survival of the seed pieces.
The main reason for stand failure is seed piece decay caused by different species of the bacterium Erwinia. Warm, moist soils are ideal conditions for Erwinia to cause decay. However, high soil temperatures alone can also reduce growth or even kill emerging stems. Soil temperatures of 86° to 96°F may stunt emerging stems while temperatures greater than 96°F may kill the stems as they push through the soil. Fortunately, several steps can be taken by growers to increase the survival of summer planted potatoes.
Freshly cut seed pieces are more susceptible to infection and have a much higher rate of respiration compared to seed that is cut and allowed to heal. Healed seed pieces produce a corky layer of suberized cells over the cut surface that provides a physical barrier from infection by bacteria and fungi. After healing, respiration rates of cut seed pieces slow to a lower and more even level. A high respiration rate means more oxygen is needed by the seed piece to stay alive and grow. If the soil is too wet, the exchange of air between the soil and atmosphere may be too slow to meet the demands of freshly cut seed pieces. Therefore, for disease resistance and reduced respiration rates, only healed seed pieces should be used. Well-healed seed pieces are produced by placing the cut seed in bins and holding them for three to six days in surrounding air of 55° to 70°F with a relative humidity of at least 90%. The potato pieces will then form a corky layer over the cut surfaces and the respiration will slow.
Besides slowing the growth of the seed piece, saturated soils cause a film of water on the surface of the seed piece which favors the growth of bacteria such as Erwinia. Fields should be pre-irrigated before planting. Trying to irrigate a newly planted potato field will only increase the amount of seed piece decay.
Seed pieces do best in summer plantings when they are planted a depth of six inches. At this depth the soil moisture, rate of air exchange, and temperature favor the growth of the seed piece and not soft rot organisms.
In August, soil temperatures are lowest between 4 and 8 a.m. and increase again at 10 a.m. Planting during this coolest period will cause less entrapment of heat around the seed piece as it is covered with soil, which allows the seed to start at more ideal conditions. Sprinkler irrigation can also be used before plant emergence to cool the soil, but it should be limited to duration of only 15 to 30 minutes. The objective will be to wet only the surface of the soil and not to add extra moisture around the seed piece. As the water evaporates from the soil surface, energy (in the form of heat) will be used. The soil can potentially cooled by 8 to 16 degrees at the one-inch depth and 5 to 12 degrees at the six- inch depth.
Finally, changing the planting dates slightly may have a big impact on the survival of the seed pieces. Based on the 30-year weather average for Kern County, the hottest period has been from July 23 to August 11 with highs ranging from 99° to 101°F. After August 13, the normal highs are 96°F or less. Waiting to plant until August 13 might increase the chance of seed piece survival.
In summary, although summer potatoes can be a difficult venture for growers, there are some measures growers can use to lower the risk of planting in less than ideal conditions. These include the use of well-healed seed and planting in pre-irrigated beds at the six-inch depth. Planting in the early morning hours will entrap less and sprinkle irrigated only for a very short period of time before emergence to cool the soil. Finally, if possible, plant in the second half of August when historically the air temperatures are slightly cooler.