First thought is concern about lack of moisture and excessive heat
Next thought is frequency of crop rotation
and lack of resistance to 'Fusarium wilt' which is common to heirloom veggies.
I water thoroughly twice a week until the soil is saturated. Heat? I have a 30% shade cloth over the hoophouse which did well for the past 2 years. This year has been odd though. Nothing seems to grow and produce as vigorously as the two previous years. Can't do much about the heat though.
Crops are rotated through different beds and by the seasons with the addition of fertilizer and new manure to fr4shen them up. The winter crops will be heavy with collard, kale, spinach, carrots, radish, etc.
'Fusarium wilt'? I'll have to research that. Thanks!