|March 2009. Lagenaria siceraria (bottle gourd), variety Pusa summer prolific, grown hydroponically at Mochudi, Botswana, is showing chlorosis, partial necrosis and inward curling of the leaves. Plants were negative when tested for CMV and an organophosphate insecticide and dithane are added to the tank mix. Some aphids are present.
In answer to a member's question, the new leaves are healthy: it's the old leaves that are showing the symptoms. This was important to know as one of the key factors of deficiency symptoms is the mobility of nutrients within the plant. For example, nitrogen is not as mobile as phosphorus (within the plant), which means that younger leaves show earlier symptoms of chlorosis than the older leaves in the case of nitrogen deficiency, and vice versa for phosphorus.
Each of the macro and micronutrients produce symptoms differently. Generally speaking, unless you have quick access to a laboratory, a recommended fertiliser dose and a one-off spray of micronutrient mixture should solve the problem (if that is really the problem and there is no other pathology involved).
The following is a useful articles: http://4e.plantphys.net/article.php?ch=t&id=289.
There was also the suggestion that the symptoms were due to Mg deficiency. If nutrients are being supplied in quantities proportional to plant uptake, external Mg concentration could be too low, and a high K:Mg ratio inhibits Mg uptake. Is the solution pH controlled? Acidification could exacerbate a Mg deficiency.
If it is late-onset, it's possible that it is K deficiency, with the increase in demand for K at the fruit-filling stage, resulting in remobilization from older leaves. Cotton is prone to this. The greatly reduced size of the fruit would be consistent with this, although Weir and Cresswell suggest K deficient cucumbers fill at the blossom end and are narrow at the stem end, which is the opposite of the symptoms in the picture. It's a very dramatic chlorosis and relatively little necrosis for K deficiency (hence more likely Mg deficiency), but acute onset at fruit filling might cause this.
The darkness of the necrotic tissue is also more like K deficiency than Mg - but that's quite species dependent, depending on tannin content etc, and I'm not that familiar with cucurbits. However, I would have expected some symptoms on the younger fully expanded leaves, if it were K deficiency: a fine light-green mottle, or speckling of small necrotic lesions in interveinal pockets.
If it's Mg deficiency, chlorisis should be worse on the sunny side of the plants (due to photobleaching). No difference for K deficiency.