Feature Articles

You're the Expert
Are you tuned in to your crops' nutritional needs? See how you score with this quiz.

Doug is the head grower at a mid-sized greenhouse. Things have been going well all season. So well, in fact, that Doug decided to take off for a long weekend before things got really hectic with shipping.

When Doug returned on Monday morning, he had a significant problem. The lower leaves on all the ivy geraniums in one greenhouse range were dead or in the process of dying back. The 4-inch New Guinea impatiens that were transplanted two weeks ago and placed under the baskets appear to be growing normally.

Doug took photographs, a quick pH and EC test from the media of several of the affected geranium baskets [pH =5.7 (optimum 5.8 to 6.4), EC+0.3mS/cm (optimum 0.6 to 1.5mS/cm)] using a 1 soil:2 water test. Doug then emailed several experts for help, noting that the plants are being fertilized with 100ppm N from 20-10-20 with each irrigation. Within two days, he received the following diagnoses and recommendations.

Expert 1. The problem appears to be ethylene damage. Ethylene is a gas, usually produced by the incomplete combustion or exhaust gases being vented into the greenhouse. The symptom most commonly associated with ethylene us epinasty (downward curling of the leaves) and premature death of flowers. However, in some species, including ivy geraniums, a common symptom is chlorosis (yellowing) or necrosis (death) in the older leaves.

In the affected greenhouse, check all unit heaters and exhaust ducts for cracks or holes. Once the unit heaters have been fixed clean any dead or dying leaves from the plant.

Expert 2. The problem appears to be iron manganese toxicity. Even though ivy geraniums are less sensitive than zonal or seed geraniums (Pelargonium x hortorum) to micronutrient toxicity, toxicity can sometimes occur if media-pH gets very low. In container media, the pH will tend to decrease as the media dries. Even though the pH is within the acceptable range for ivy geraniums, according to the in-house test, as the media dried, the pH may have dropped below 5.5, allowing the plant to take up too much iron and/ or manganese.

Increase the media pH to around 6.0 using either potassium bicarbonate [at 2 pounds /100 gallons (240 grams/100liters)] or flowable limestone [at 64 fl. oz. /100 gallons (0.5liters/100 liters)] applied as a drench. Remember to wash the foliage with clear water after the application. The day after you apply the lime or bicarbonate, drench with a high nitrate fertilizer at a concentration of about 400 ppm N. Also, clean the dead or dying leaves off the plant.

Expert 3. The problem appears to be drought stress. Ivy geraniums don't wilt when their media gets dry. So when the media dries out completely, the lower leaves either get necrotic spots or they die outright.

Clean any dead or dying leaves from the plant. Also check for root damage. Even if no root damage can be seen, apply a broad-spectrum fungicide drench to protect the roots.

Expert 4. The problem appears to be low nutrition. Geraniums are high fertilizer requiring plants. When nutrient levels in the media drop too low (as in this case), the plant can start taking reserves out of the lower leaves, resulting in chlorosis and/or necrosis in the oldest leaves.

Clean any dead or dying leaves from t plant. Increase the fertilizer concentration to 400 ppm from 20-10-20 for a couple of applications before reducing to a constant 250 ppm for the remainder of the crop.

What is your diagnosis and what should be done?
So what should Doug do? He has four opinions about what caused the damage to the foliage of the ivy geraniums that are fundamentally different from one another.

The first thing would be to look for any commonality between the experts. In fact, everyone agreed that the baskets should be cleaned of dead or dying foliage. That is a good place to start because botrytis could potentially become a problem if the dead leaves were not removed from the plant.

The next step should be to systematically test all the possibilities. This is also relatively simple. For example, Do9ug could go out to the greenhouse and check the roots of the ivy geraniums for damage or disease (Expert 3). He could get a technician to come to the greenhouse the next day or two to check the heaters for leaks  (Expert 1). He could send some media and tissue by overnight mail to a commercial laboratory for nutrient analysis (Experts 2 and 4), but it will still take about 4 to five days to get the results. Finally, he should send the plants to a diagnostic lab (just in case) to check for any disease organisms on the leaves or the roots, but this will also take several days to get the results.

In checking the roots, Doug finds that many of the baskets have been damaged and the root loss is fairly uniform across the crop. Instead of waiting for the results from any of the other tests, Doug decides to drench the baskets with a broad-spectrum fungicide.

In addition, he decides to apply some fertilizer to the pots at the same time. However, Doug decides not to use the 20-10-20 recommended by Expert 4, because with 40 percent ammonium, this fertilizer has an acid reaction and would drop pH further. Instead, he uses a less acidic 17-5-17 ( with 25 percent NH₄-N) at 200 ppm N. This small changes allows him to apply some fertilizer, without a further drop in the substrate pH.

Two days later (on Wednesday), the heating technician finds that there is nothing wrong with the heaters, indicating that ethylene probably did not cause the damage.

Seven days after finding the problem (the following Monday), Doug receives the results from both the diagnostic lab and the soil and tissue tests. The diagnostic lab found pythium in the roots and botrytis on the tissue. The soil and tissue tests results were found to be within an acceptable range (indicating that iron toxicity or low nutrition) were not a problem.

Once all the test results were in, Doug concluded that the tissue damage on the ivy geraniums was probably caused by drought stress.

Conclusion
The Internet can be a wonderful tool. It allows you to access information like never before. For example, Doug would never have been able to get the opinions of four industry experts within two days before the Internet.

However, because these experts could not see the crop in person, they could not make the intangible observations that allow you to connect the dots and determine what caused the problem. Instead, the experts were left with an incomplete snapshot of what had occurred. Without much information, these experts tended to fit the problem into their own experiences or expertise, which is why Doug ended up with four fundamentally different opinions to what caused the damage to the ivy geraniums.

Given multiple reasons for a given problem, it is important to take the time to systematically prove or disprove each of them. For example, Doug found and treated for root disease, which gave weight to the argument that drought stress caused the problem. But the fact that the plants had root disease did not eliminate the other possibilities either.

Plants are poor communicators. If they are somewhat stressed, they usually turn yellow. If they are very stresses, they turn brown. It is often impossible to correctly diagnose a problem based solely on how the crop looks.

The reason we called this series "You're the Expert" is that growers must diagnose their own problems and make decisions every day. Resources, such as books, other growers, consultants and testing labs are a back-up. However, even the best experts available need information (crop observations, management history and test results) before jumping to conclusions.

About the Authors:
Bill Argo is technical manager at Blackmore Co; bargo@blackmoreco.com

Paul Fisher is associate professor and Extension specialist at University of New Hampshire (UNH); paul.fisher@unh.edu

Bruce MacKay is a senior lecturer at Massey University, New Zealand; b.mackay@massey.ac.nz

The authors thank the UNH Young Plant Center sponsors for their continued and generous support.

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