Answer: If your squash plants leaves are turning yellow, something is wrong. The hard part is figuring out exactly what is wrong. The leaves on a squash plant will start to turn yellow any time the plant is stressed. Below, I have listed a few reasons why a squash plant may be stressed.
Lack of water
While squash plants are pretty hardy plants, as far as vegetable plants go, they do need about two inches of water a week. Sometimes they will need more due to high temperatures. Check to see if your squash plants are getting at least this much water a week. If not, supplement natural watering (i.e. rain) with a sprinkler or a drip hose.
Vine borers will attack a squash plant and make its way through the vine of the plant. Tell tale signs of a vine borer include yellowing of the leaves gradually from the base end of the vine to the tip and a small pile of ?sawdust? at the base of the vine, near where it comes out of the ground. If you suspect a vine borer, be aware that pesticides will not work. The only effective, though not always successful, treatment is to try to remove the vine borer worm from the stem. Go to the spot where you suspect the vine borer is lodged and carefully slit the vine lengthwise (in the direction of the capillaries). This will not hurt the squash plant too much and either way, if you do not find the vine borer, the plant is doomed anyway. If you are able to locate the vine borer, use a tooth pick to pierce and kill it.
Without iron, plants have a difficult time making chlorophyll, the substance that makes leaves green. Adding Iron Chelates (a kind of fertilizer) to the soil can help. Most of the time, iron deficiency is a result of the nutrients being leeched out of the soil due to over watering. Make sure that you are not over watering your plants.
Unfortunately, if your squash plants are infected by bacterial wilt, then there is nothing you can do to save them. The yellowing of the leaves will be followed rapidly by wilting and browning of the leaves and eventually death. Bacterial wilt can be diagnosed by cutting off a piece of the stem and squeezing out some of the juice inside. If the juice comes out slimy or oozing, than the plant has been infected. Destroy the plants and do not compost them. Do not plant squash or other cucurbit vines in that location next year, as the bacterial wilt will still be in the soil and will infect them as well.
While the conditions listed above are some of the most common reasons for squash plants developing yellow leaves, they are not the only ones. As stated above, the leaves on squash plants will turn yellow anytime the plant is stressed. If you can find out what is stressing the plant, than you will be able to remedy the situation and help your squash plant regain it green coloring.
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