Resource Guide for Organic Insect and Disease ... - Cornell University

MATERIAL FACT SHEET
Sulfur
MATERIAL NAME: Elemental sulfur, Lime sulfur
MATERIAL TYPE: Element
U.S EPA TOXICITY CATEGORY: Elemental sulfur: III, “Caution”, Lime sulfur: I, “Danger”
USDA-NOP
Both sulfur and lime sulfur (aka calcium polysulfide) are considered to be allowed, synthetic
materials; their use is regulated in certified organic production. Preventive, cultural,
mechanical, and physical methods must be first choice for pest control, and conditions for use
of a biological or botanical material must be documented in the organic system plan (NOP
2000).
MATERIAL DESCRIPTION
Sulfur is one of the oldest known pesticides, cited by the Greek poet Homer in approximately
1000 B.C.E for its “pest averting” properties (Horsfall 1945). In its native state, it is a bright
yellow crystalline solid. It was first recommended for control of plant diseases in the early
19 th century and was widely used until the introduction of Bordeaux mixture in the 1870’s.
For a time, Bordeaux was the main fungicide until sulfur regained its position in the early 20 th
century. It is still widely used against powdery mildews and mites in grapes and other crops.
Lime sulfur is a mixture of calcium polysulfides created by boiling lime and sulfur together. It is
an orange liquid with a strong rotting egg odor. It is also one of the earliest known fungicides,
used against peach leaf curl in 1888 (Horsfall 1945). Lime sulfur is more toxic to fungi and has
more potential for phytotoxicity than elemental sulfur.
HOW IT WORKS
As a fungicide, both sulfur and lime sulfur inhibit spore germination and growth in a number
of fungal plant pathogens, likely by permeating the cell wall and interfering with important
metabolic processes (Williams & Cooper 2004). Sulfur is a protectant; therefore, it must
be applied before the pathogen comes into contact with the leaf in order to be effective.
However, lime sulfur has a “kickback” effect against apple scab, providing some control
when applied within 72 hours after an infection period. Sulfur has also been identified as a
component of the induced defense mechanisms of some plants, accumulating in the xylem at
levels toxic to some vascular pathogens. Sulfur fertilization has been associated with increased
host resistance in some host/pathogen interactions (Williams & Cooper 2004). Soil application
of sulfur, either as ammonium or potassium sulfate or elemental sulfur, has been used to
reduce scab (caused by Streptomyces scabies) and black scurf (Rhizoctonia solani) on potato,
Rhizoctonia root rot in beet, and Streptomyces soil rot on sweet potato. The mechanism for
control has generally been thought to be the acidifying effect of sulfur and the intolerance of
these soil-borne pathogens to low pH (Oswald & Wright 1950). More recent studies suggest
that sulfur also increases microbial biodiversity and antibiosis (an association between two
organisms that is detrimental to one of them) against Streptomyces scabies in the soil (Sturtz
et al. 2004) and increases host plant resistance to Rhizoctonia solani in potato (Klikocka et al.
2005).
As an acaricide (mite-killer), the mode of action of sulfur is not well understood. Its impact on
pest mites is influenced by temperature and relative humidity, with efficacy increasing with
higher temperature and relative humidity (Auger et al. 2003).
172 Organic Resource Guide