Potato Blight

Prior to vacation there were news reports on Blight... in regard to Potatoes... of course the first year we tried to plant them the weather conditions were ideal... and with Mom expressing her concern... on Vacation day 4 out came the potatoes... and yes there were really potatoes at the end of those huge plants... but the blight can spread to tomatoes and around the neighborhood... and there are huge gardens in the neighboring yards with tomato plants so it had to be done... maybe next year!



Late blight may appear on leaves, stems, and tubers. Leaf symptoms appear as pale green water soaked lesions. These lesions are usually at the tips or margins of the leaves, but not always. The lesions grow into brown or purplish-black lesions which sometimes have a yellow halo. The halo is not always present in late blight infections. During humid conditions a white growth of mycelium and spores can be seen on the underside of the leaves. Lesions also appear on the petioles and stems as black, greasy areas. Stem lesions may girdle the stem and kill the foliage above the lesion. Stem lesions were prevalent during the 1995 season in Malheur County and Southwestern Idaho.

A field severely infected with late blight will give off a characteristic foul odor caused by rapidly decaying potato tissue.

Environmental conditions must be conducive to disease development before the disease can develop. Humidity needs to be 90% or greater for spore development, conditions which can occur frequently inside the potato canopy. Temperatures below 780 F are necessary for the spores to develop. Free moisture must be present on the plant in order for the spores to germinate and infect a new plant. Infection requires cool days to keep evapotranspiration low and frequent rainfall or overhead irrigation or a combination of both to provide long periods of free moisture over a 3-5 day period of time. To recap, the following environmental conditions are necessary for late blight development:
TEMPERATURES- Below 780 F
HUMIDITY- 90% and higher
MOISTURE- free moisture for 8-12 hours