In some regions, recent excessive flooding has been devastating: preventing planting, washing away soils, causing concerns with water and soil pollution, decimating crops, displacing livestock and claiming lives.

With much of the nation seeing overly wet weather during this spring, many growers are contending with a variety of problems that can arise when there is too much of a good thing: moisture. Water-logged soils and wet leaves can all play a role in vegetable diseases.

The onset of drier weather conditions during the summer growing season can often stop virulent diseases, which have the potential to otherwise become severe. Many diseases which require moisture in the soil or on plants’ leaves in order to complete their infection cycle can, however, re-infect plants. This accumulated disease pressure can exacerbate symptoms if wet conditions continue throughout the season, or later re-emerge.

Wet weather also causes the proliferation of abiotic disease concerns. These vegetable diseases aren’t caused by an infectious agent but by environmental conditions. Lack of soil oxygen, depletion of nutrients from excessive soil moisture or changes in soil pH can all negatively impact plant health and crop yield without any infectious agent being present. Less than ideal growing conditions cause plants to become more susceptible to any diseases they do encounter.

Areas receiving excessive rainfall can have soil leaching, as calcium, magnesium, sodium and potassium are depleted from the soil, increasing its acidity. They are replaced by more acidic cations. Such changes in pH can drastically effect plant growth and health. While this pH change doesn’t occur instantaneously, excessive soil moisture can cause pH concerns in a wet growing season.

Sandy soils and poorly drained soils are most prone to nutrient leaching from excessive wet weather. Leaching means crops won’t have the fertility they need to grow and thrive. Denitrification, caused by nitrogen leaving saturated soils and entering the atmosphere as a gas, is a concern under excessively wet soil conditions.

Soil erosion and compacted soils are other woes of excessively wet weather. Heavy rain events themselves can physically damage delicate plant tissues.

Signs of Distress

Nitrogen deficiencies in vegetables often cause yellowing of the leaves. Plant yellowing is also caused by fungal diseases, and wet conditions are required for most of these to proliferate.

Wet soils can also cause wilting. Wilting can be due to a lack of oxygen in the soil. As water clogs up the air spaces it causes root hairs to die. If the soil dries out enough before two or more days pass, the plants can revive.

But wilting under wet conditions can also indicate root rot diseases. Most crops are susceptible to root rot organisms, which are primarily (but not exclusively) water molds or fungi. They thrive in the soil and attack roots when soil moisture levels are high, causing low oxygen or even anaerobic soil conditions. As the roots drown, these opportunistic organisms are able to enter the plant roots. Spores which move through water help to further spread these diseases.

Bacterial pathogens are diverse, and many do require wet conditions. Prolonged leaf wetness can cause leaf spot, while excessive soil moisture favor bacterial root rots.

Wet weather also brings out some unwelcome pests. Slugs favor wet soils and frequent rainfall and can quickly damage crops. Pollination can be impacted as honeybees don’t fly with rain or mist in the air, or in temperatures much below 60º F. Bumblebees can work under these conditions, however.

So far, this growing season’s weather may have farmers frustrated. Delayed planting or slow plant growth due to wet and cool soil conditions have been frequently seen in many regions. Remaining alert for diseases and issues which normally aren’t a concern in their fields, but which thrive under the existing weather patterns, is prudent.