Flooding Injury and Subsequent Disease Infection of Fruit Crops

Published: June 5, 2019

Fruit crops have a wide range of tolerance for flooded or waterlogged soils. Roots of peach and apricot trees are highly sensitive to waterlogged soils, roots of cherry and plum trees are intermediate, and those of apple and pear are the least sensitive. For small fruit crops, the most sensitive to flooding are strawberry (can tolerate submersion in water for up to seven days), blackberry, raspberry and currants, while blueberry, grape, and gooseberry are relatively more tolerant of waterlogged soils. However, after the flood water recedes, Phytophthora root rot may occur on some crops. Although fruit trees may produce a crop in the year of the flood, they may still die the following growing season when subjected to stress.

Shortly after flooding, gas exchange between the soil and air is greatly reduced. Microorganisms consume much of the oxygen in water and soil. The lack of soil oxygen induces several changes in plants and soils that adversely affect vegetative and fruit growth. Plants respond by reducing water absorption and closing their stomata, resulting in a lower rate of photosynthesis. After this, root permeability and mineral uptake is reduced. In waterlogged soils, reduced ions (NO₂^-, Mn²⁺, Fe²⁺, S^-) and phytotoxic byproducts of microbes can accumulate and restrict root metabolism. Leaching and denitrification may also occur.

The first plant symptoms usually observed after a short period of flooding are wilted leaves and an unpleasant odor that often emanates from the injured or dead roots, which appear brown. Other plant symptoms of waterlogging include leaf epinasty (downward bending and twisting), chlorosis, or abscission. Reduced foliar and fruit growth may also occur and roots may fail to regenerate. Hypertrophied lenticels (white, spongy tissue) may also be present on the underground rootstock shanks of fruit trees.

Summer floods cause greater root damage than those in the spring. Also, the higher the soil temperature, the greater the plant injury. Flooded peach trees can wilt within six, ten, and sixteen days when the temperature is 80, 71, and 63°F., respectively.

To reduce damage to flooded fruit plantings, drain the field as quickly as possible by pumping or digging new drainage channels. If a new layer of silt or other debris has been deposited over the original soil surface, remove it and restore the soil surface to its original level. If the soil has eroded, fill in these spots with the same soil type that is in the orchard. Do not use sand, mulch, or other soil types that will alter the internal drainage properties or texture of the existing soil when adding soil around plant roots. When the soil is still moist, stake any trees that have fallen over. Do not fertilize fruit crops after July 1 as fertilization after this date often promotes late season growth that results in winter injury. For commercial fruit plantings, fungicides may be applied after the flood water has receded and it is determined and that fruit crops are still alive. Fungicides applications for commercial producers can be found at: https://ag.purdue.edu/hla/Hort/Documents/ID-465.pdf.

Plants that survive summer flooding are weakened and more susceptible to diseases, pests, and other environmental stresses. Check fruit crops for Phytophthora root rot symptoms. Blueberry plants infected with this disease often have yellow leaves that appear "burned" at the margins and may prematurely defoliate in the fall. Waterlogged brambles may also become infected with Phytophthora, especially red raspberry. Canby, Hilton, Ruby, Polana, and Titan red raspberry are highly susceptible to Phytophthora, Encore, Festival, Heritage, Reveille, and Taylor are moderately susceptible, and Jaclyn, Latham, Boyne, Killarney, Nordic, and Prelude are more tolerant to root rot. Black raspberry cultivars, such as Bristol, Dundee, and Jewel are considered moderately to highly resistant to root rot and blackberry cultivars are also relatively resistant to Phytophthora. Since this fungus can persist in the soil for many years, look for stunted fruiting canes with weak lateral shoots and leaves with interveinal and marginal chlorosis next spring. When the epidermis is scraped off the crown or roots, a distinct line between healthy (white) and infected (reddish brown) tissues is visible on infected tissue.

Figure 1 Red stele disease symptoms caused by Phytophthora fragariae on strawberry roots.

Leather rot is another disease caused by Phytophthora fungus that affects strawberry crowns and/or fruit. Infection can occur during flowering or after fruit set. The affected areas appear brown to black on immature fruit (Figure 1). Mature fruit appears bleached and the color ranges from light tan to light red. The flavor of the infected fruit is bitter and the texture is tough. When Phytophthora infects the crown, leaves will often pull away from the crown when the plants are lifted. The vascular tissue appears necrotic when the crown is bisected.

Figure 2 Leather rot symptoms caused by Phytophthora cactorum on strawberries.

Red stele root rot, which is caused by a different species of Phytophthora than leather rot, can be problematic on strawberry following flooding. Symptoms of red stele include wilted foliage during hot weather and stunted foliage. When roots are cut longitudinally, a reddish-brown discoloration is visible in the stele (central core) of the young root above the rotted tips (Figure 2). This disease reduces fruit size and yield. Strawberry cultivars, such as Earliglow, Allstar, Guardian, Lateglow, Lester, Midway, Redchief, Redglow, Sparkle, Sunrise, and Surecrop are resistant to common types of red stele fungus, whereas Honeoye, Jewel, and Cardinal are susceptible.

Phytophthora crown and root rot occasionally affects grapes. When vines are infected with this fungus, the foliage appears chlorotic. Cankers develop on the trunk of the vine near the soil surface and the bark often sloughs off. Underneath the bark of the trunk or on the roots, the woody tissue is black and decayed. Vines severely infected with Phytophthora usually die, but those with a low level of infection can produce new tissue, and the vine will recover.

Figure 3 Phytophthora crown rot on trunk of apple tree.

In fruit trees, Phytophthora infection causes collar or crown rot. Symptoms of this disease are similar to those induced by flooding in that foliage of infected trees is generally sparse and chlorotic. However, the two problems can be distinguished by examining the trunk and root tissues. When trees are waterlogged, the roots are often black and have a foul odor. However, with crown rot, the bark is killed in a band at or near the soil surface (Figure 3). Disease symptoms also occur below several inches below the soil surface on the trunk and roots with infected tissue appearing reddish brown whereas healthy tissue is white. Apple rootstocks, such as MM.106, B.118, M.7, and M.26 are susceptible to Phytophthora, while MM.111, M.27, M.9, B.9, P.2, P.22, and the Cornell/Geneva series (G.11, G.30, G.41, G.65, etc.) are considered more resistant to this fungus.