Current weather forecasts predict the San Joaquin Valley may experience alternating periods of cool and warm temperatures in the next few weeks. Such conditions may lead to the development of weather-related disorder known as “spring fever”. On vines with spring fever, basal leaves will become chlorotic, with green leaf-color fading first from the leaf margins, and then progressing inwards, towards primary and secondary veins (Figure 1 shows a variety of spring fever symptoms). Also, leaf margins will curl upward, and they may become necrotic. Severely affected leaves may fall from the vine. As the weather becomes warmer and less variable—typically before bloom—the vines will grow out of this condition, normally without any lasting effects on productivity.
The symptoms of spring fever resemble those of potassium deficiency, so this disorder is sometimes referred to as “false potassium deficiency”. The reason why spring fever and true potassium deficiency induce similar leaf symptoms may be that both can lead to an excessive accumulation of the amino acid putrescine in leaves. However, symptoms of spring fever and true potassium deficiency differ in the time of the season when they are noted, and on the leaves in which they affect. As noted earlier, spring fever symptoms affect basal leaves, and symptoms will usually disappear by bloom, whereas symptoms of potassium deficiency usually begin on leaves from the middle of shoots, no earlier than bloom, and extend to younger leaves. Moreover, vines afflicted with spring fever are not potassium deficient, so mineral nutrient analyses of petiole samples can be used to distinguish between spring fever and potassium deficiency, if there is any doubt. The critical values for bloom samples are 1.0% K or less (deficient) and >1.5% K (adequate).