Summer 2014: Drought exacerbates root rots

Regional Report for Ventura County by Jim Downer

The drought has firmly set in for the third and historically driest year in California history. Drought affects not only plants in production but in landscapes as well. Sudden and long-term weather changes can cause physiological harm in many plants resulting in damage symptoms. Dehydration during drought leads to physiological wilting but can also be associated with sunburn of leaves or stems of many ornamental plants. Plant disfiguration from leaf drop and dead leaves, twigs and branches increase on drought-stressed plants. Drought can occur in nurseries if temperatures suddenly increase or wind events create greater transpiration resulting in water deficits. The immediate effects of drought are easily seen in aboveground plant parts, usually as a loss of bright color, and ultimately wilt, sunburn or death of leaves and small stems. Many plants don’t wilt, so they can become very dehydrated before other symptoms are noticed.

Root systems are also affected by drought. Drought causes chemical, physical and biological changes of soils that ultimately affect the health of the root systems and thus the plant’s susceptibility to Oomycete root rots. As irrigation water is removed from media by plants and evaporates from media or soil surfaces around plants, salts are left behind in soil. Each year rainfall can potentially leach accumulating salts, but rainfall volumes need to be large enough to move water down in the soil or media profiles while also dissolving salts and flushing them from the root zone. These last three years have not provided any significant leaching opportunities. Large containerized trees and landscape plants are all affected by the resultant increasing soil osmotic potentials (higher salt content) which accompanies drought. Nursery growers may need to develop an effective leaching program to prevent excessive accumulation of salts in the root zone (editors note: see Richard Evans feature article “Leaching to manage salinity in ornamental crops”).

Chemical factors in soil have long been known to affect the outcome of Phytophthora diseases (Schmitthenner and Canaday 1983). Drought-affected plants growing in saline soils have less ability to regulate the influx of salts entering their root systems because roots growing in saline soils are more likely to have increased exudates that are attractive to the spores of Oomycete pathogens which further disable root ion selectivity. Specific ions can alter the progress of many Phytophthora diseases either increasing or decreasing them. While chlorides and sodium in solution are root rot predisposing (MacDonald and others 1984), calcium ions (Ca⁺⁺) have been shown to control root rots by interrupting the swimming ability of zoospores and by decreasing sporangial volume, thus decreasing inoculum potential (Messenger et al. 2000). Unfortunately, many waters in California are alkaline and thus cause calcium to precipitate from solution as limestone minerals. Schmitthenner and Canaday noted that pH reduction (below pH 5) was associated with reduced incidence of disease from most reported Phytophthora species. Therefore, decreasing irrigation water pH and adding calcium ions may be an effective control measure against Phytophthora in nurseries.

Nursery growers are especially aware that peat moss and other highly organic media can dry out and become hydrophobic. This physical change in the media repels water, making the media hard to rewet and difficult to manage. Severe soil moisture deficits may cause plants to permanently wilt and also affect the soil microbial community (soil food web). Soil microbes such as mycorrhizae, plant growth promoting bacteria and saprophytic fungi that are also fungal hyperparasites of Phytophthora, all have a role in regulating soil-borne diseases (Hornby 1990). In dry soils, many microbial partners are lost or die off — the food web simplifies, loses resiliency and is open to the effect of opportunists (plant pathogens) that can rapidly invade soils when moisture levels come back to normal and disease begins.

Jim Downer
Environmental Horticulture Advisor
UC Cooperative Extension Ventura County
669 County Square Drive, #100
Ventura, CA 93003-5401
(805) 645-1458 phone, (805) 645-1474 fax
ajdowner@ucanr.edu
http://ceventura.ucdavis.edu

References

Hornby D. 1990. Biological Control of Soil-Borne Plant Pathogens. Wallingford, UK: CAB International. 479 p.

MacDonald JD, Swiecki TJ, Blaker NS, Shapiro, JD. 1984. Effects of salinity stress on the development of Phytophthora root rots. Calif. Agriculture 38:23-34.

Messenger BJ, Menge JA, Pond E. 2000. Effects of gypsum on zoospores and sporangia of Phytophthora cinnamomi in field soil. Plant Disease 84:617-621.

Schmitthenner AF, Canaday CH. 1983. Role of chemical factors in development of Phytophthora diseases. In: Erwin DC, Bartnicki-Garcia S, Tsao PH (eds). Phytophthora Its Biology, Taxonomy, Ecology, and Pathology. St. Paul Mn: American Phytopathological Press. p 189-196.