Answer: If these are new plants, browning can be caused by transplant stress, or due to poor rooting and/or poor watering practices. Poor rooting can be related to planting rootbound plants without cutting or untwining encircling roots, or to planting into poorly prepared soil. Planting this late in the season can limit rooting due to the heat and related stress on the plants.
Incorrect watering can also limit rooting and both over watering and under watering can cause drooping or wilting and foliage damage due to root damage. Your goal in watering is to keep the soil evenly moist like a wrung out sponge, not sopping wet and not dried out. To know if you need to water, dig into the soil with your finger. If it is still damp, do not water yet. When you do water, apply it to the soil surface and water thoroughly and slowly so it soaks down to the deeper roots. After watering, wait a few hours and then dig down to see how far the water soaked in; it can be surprising.
There is no set schedule for watering, it depends on your soil type and on the weather. Using an organic mulch several inches thick over the root area will help reduce watering needs as well as feed the soil gradually as it breaks down over time.
Based on your description and the time of year, it's possible this may be a fungal disease problem. Make sure your plants are adequately spaced to allow good air circulation. If you water, avoid wetting the foliage. Remove the worst affected leaves and clean up any dropped leaves and dispose of them in the trash.
If the problem continues, I would suggest you contact your local Cornell county extension to obtain a more specific diagnosis and based on knowing that, decide what to do. If a chemical control is needed, they will have the most up to date information on what to use and how/when is best to use it for maximum results.
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