Answer: The roots, bark, and leaves of black walnut (Juglans nigra) exude juglone. Some plants are more susceptible to this growth inhibiting toxin and will not grow directly beneath the canopy of walnut trees. How close you can plant to walnut trees really depends upon soil type and drainage. Small amounts of juglone are released by the tree roots, but a far greater amount of this growth inhibiting chemical is found under the canopy of the tree. Many plants are sensitive to juglone, including apple, blackberry, pear, blueberry, asparagus, cabbage, eggplant, pepper, potato and tomato. So this could be affecting your eggplant.
Another possibility is that eggplant thrives in warm soil. They need at least 6 and preferably 8 hours of direct sun a day. If your soil temperatures aren't warm, trying using black plastic mulch to warm the area. As for the insect holes, examine the plants carefully to see what's causing the problem. Holes are generally caused by chewing insects, such as caterpillars and worms, which you should be able to spot and handpick. The soapy water spray is also good for smaller insects, but you may need to do it daily to keep them under control. Insects are actually drawn to stressed plants, so the best "cure" is to keep plants thriving. If you think the walnut tree is interfering, perhaps you can transplant the eggplant elsewhere. Good luck!
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