Tips for Using Ethephon Effectively

by Mike Murray, Farm Advisor, Colusa County

There are times when processing tomato producers want to advance the crop maturity. One option is the use of ethephon, which breaks down in the plant to the naturally occurring plant hormone ethylene. Timely applications of ethephon under specific conditions can result in earlier fruit maturity, but may not be the appropriate strategy in every situation. Field tests to identify varietal responses, determine optimal spray rates and evaluate other important parameters have been conducted throughout California processing tomato production regions over the past ten years. This is a good time to review what we have learned.

Plant Health
In order for ethephon to function properly, the tomato plants must be healthy and not under stress. Factors such as drought, insect or disease injury, nutritional disorders, etc., render the plant unsuitable for ethephon applications. Avoid treating stressed or damaged fields with ethephon as there is a high likelihood of causing additional plant injury and fruit yield losses, often exceeding any maturity benefits received.

Application Timings
A common mistake is treating the plants too late. The most practical way to determine the optimal time is to shake all of the fruit from several plants chosen randomly from different representative parts of the field. The fruit should be separated into three groups: red/breaker, green, and cull fruit.
1) Include fruit with any color (pink, etc.) into the red/breaker group.
2) Only put “mature-green” fruit into the green group. Mature green are typically described as fruit that do not exhibit any color, have gel formed in the seed cavity and the seed can not be cut when slicing the fruit.
3) Put immature green (i.e. green fruit, but no gel formation in the seed cavity and seeds can be cut) and any damaged/diseased fruit into the cull group.

Weigh the three groups. When you find 5-15% red/breaker fruit (by weight), the field is ready for an ethephon application. This is considered the optimum time.

Tomatoes normally ripen at a rate of 2½ - 3% fruit per day. So a field that has 10% red fruit needs to ripen an additional 80% to be harvested at 90% maturity. This takes 27-32 days to occur “naturally”. However, a timely ethephon application can advance harvest 6 to 10 days.

It appears that many local fields are treated with ethephon considerably later than this optimum. I have observed fields treated at 35-50% red/breaker fruit. If left on their own, these fields will reach 90% maturity in 13-18 days. Although ethephon applications may advance the maturity several days when treated late, the dramatic effects noted with an earlier spray will not be obtained. The plant simply does not have the ability to respond fast enough when fields are treated too late.

Ethephon decisions must be made up to a month before anticipated harvest to attain the maximum benefits of the material.

Rates
Ethephon is labeled for use on tomatoes at rates ranging from 1¼ to 6½ pints of material per acre, depending on the season and location. High temperatures immediately following an ethephon application can result in plant injury, premature defoliation and sunburned fruit. High temperatures are poorly defined for these purposes, but sustained maximum air temperatures in excess of 95"F can be problematic. Tests have indicated that early-season application of more than 1½ pints of product per acre may result in crop injury. At these rates, maturities may be advanced by 6-10 days, if treatments are made at the 5-15% red/breaker fruit stage, as outlined above. Higher product rates can cause additional maturity, but the risk of injury increases substantially.

Varietal Effects
It is clear that different varieties respond differently to the same rate of ethephon. In one field test, fruit maturity was advanced 4 to12 days, depending on the variety. Additionally, some varieties have lusher foliage, which helps minimize fruit sunburn. The bottom line is to know your variety and how it responds to ethephon before making large-scale applications. Always be prepared for a timely harvest, since the field-holding capabilities of many varieties are reduced when treated with ethephon.

Miscellaneous Factors
Evening vs. morning applications: Tests have shown that ethephon applications cause less crop injury if made going into a cooling trend (i.e. in the evening) than if made going into a warming trend (i.e. in the morning). So always try to apply in the late afternoon or evening.

Surfactants: The question of adding surfactants is occasionally asked. I don’t have a definitive answer, but will note that the efficacy of ethephon is increased when foliage coverage is improved. However, a safer way of accomplishing good coverage may be the use of higher water rates and/or improved spray delivery technology instead of using a surfactant. Applications with ground rigs and directed spray nozzles generally result in better plant coverage than aerial applications.

Timely harvest: Plants treated with ethephon do not hold as well in the field compared to untreated plants, so timely harvesting is needed to prevent fruit decay and losses. The maturity process that ethephon triggers ultimately results in senescence and fruit breakdown, so fruit needs to be removed and processed in a timely fashion.

Fruit color: There is a question of the effects of ethephon on fruit color. My experience has been that ethephon does not alter the color of sound fruit. However as previously mentioned, ethephon applications must be timely for several reasons. High temperatures immediately following application and/or applications to stressed plants can result in premature plant defoliation and sunburn. Poor color in loads treated with ethephon result from sunburned fruit being mixed into that load. The sunburned fruit is co-mingled with the undamaged fruit and the fruit color of the load is graded lower in color.

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