Diagnosing Yellowing in a Pumpkin Field
A grower calls about 8 acres of pumpkins. For two weeks the vines have shown yellowing that starts at the leaf margins and moves inward, with some interveinal chlorosis on older leaves first. The field was in sweet corn last year and received a broadcast pre-emergent. Soil pH is 6.8, and a recent tissue test shows low potassium and adequate nitrogen. The grower also reports wilting during midday heat that recovers at night, and a few plants have collapsed entirely with a soft, dark rot at the crown. Irrigation is drip, and the field has a history of Phytophthora in wet years. The grower wants to spray a foliar fungicide immediately and add a high-nitrogen foliar feed.
1. Separate the likely causes and explain what symptom pattern tells you what.
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There appear to be at least two problems, and treating them as one would waste money. Pattern A - marginal yellowing moving inward plus interveinal chlorosis on older leaves, with low tissue potassium - fits potassium deficiency. Older leaves first is the key: mobile nutrients (K, N, Mg) show on old growth, immobile ones on new growth. Pattern B - midday wilt that recovers at night, plus crown rot and a Phytophthora history - fits a root/crown pathogen, not a nutrient problem. Midday wilt that recovers means the plant cannot move enough water; combined with crown rot, that points to compromised roots. The high-nitrogen foliar feed is a poor idea: extra N can push lush growth that worsens both K imbalance and disease susceptibility, and it does nothing for the crown rot. The foliar fungicide may help only if the pathogen is foliar and the product is labeled for it - it will not rescue crown rot already present.
2. What should you recommend before any spray goes out?
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Confirm the diagnosis before spending on chemistry. (1) Dig up symptomatic and healthy plants and examine roots and crown - look for root rot, crown lesions, and vascular discoloration. (2) Send a sample to a diagnostic lab to confirm Phytophthora or another pathogen; do not guess between Pythium, Phytophthora, and Fusarium. (3) Check soil and tissue potassium against the deficiency threshold and review the pre-emergent history for any carryover or root-inhibiting effect. (4) Map where symptoms are worst - low, wet spots favor Phytophthora, uniform areas favor nutrition. Only then choose a labeled fungicide matched to the confirmed pathogen, applied per label timing and resistance-management guidance, and correct the potassium with a soil-applied program rather than a foliar feed. Document the reasoning so the grower sees why the immediate spray was deferred.