Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
In troubleshooting vegetables in the summer months, we see fields where the major symptom is an overall lack of vigor and this poor vigor is due to one or more stress factors. Hot weather makes this stress more pronounced. Pests such as root and crown rot fungi, bacterial and fungal wilt organisms, and insects such as squash bugs can damage plant roots, stems, and vascular systems, limiting water uptake, and causing excess stress. However, there are many stresses that don’t involve diseases or insects. The following are some other causes of excess stress in vegetables this time of year.
Soil Compaction
Plants will have limited rooting in compacted areas and therefore cannot take up adequate water or mineral nutrients. In addition, compacted soils have reduced air exchange. Plants will often be stunted and will wilt early in the day in high temperatures. Cultivation can alleviate surface compaction but will be ineffective on deeper compaction.
High Soil Temperatures
Soils that have limited water holding capacity can have excessively high soil temperatures during long hot days in late spring and early summer. Late planted crops on black plastic mulch are very likely to be exposed to high soil temperatures and surface roots will often be damaged. Overhead irrigation over the black plastic mulch is very beneficial to reduce heat loads until plants have sufficient canopies to shade over the mulch.
Drip Tape and Drip Irrigation “Diseases”
Issues with drip irrigation can often be the cause of plant stress due to inadequate water. This includes plugged emitters; leaks due to insect or animal chewing that limit water flow further down the tape; leaky connections reducing flow; tape twisting and binding, again limiting flow past the point of the bind,; improper tape selection or improper irrigation timing leading to under application of water; limited well capacity also leading to under application of water; too wide of emitter spacing for the crop or soil; too wide of bed for a single tape (with double rows) and others drip irrigation problems. Over application of water in drip irrigation also can be an issue, especially in lower field areas and where soil types change in the field. This can lead to saturated beds limiting oxygen for roots. The keys to avoiding drip irrigation associated problems is to monitor fields closely, note any areas that look stressed, and investigate whether or not the drip irrigation is functioning properly. Soil moisture monitoring devices can aid greatly in detecting problems.
Inadequate Overhead Irrigation
Under-watering can lead to additional plant stress. Plugged nozzles are a major problem that often goes uncorrected. Excessive runoff due to compacted soils can lead to reduced water intake.
Excessive Fertilization
Salt induced stress conditions can occur when excess fertilizer, manure, or high salt compost is applied or when high salt index fertilizer is applied too close to vegetable plants.