4 diseases to watch for in summer

Summer diseases develop very fast and are often hard to diagnose through symptoms alone. Phytophthora, Rhizoctonia and Cylindrocladium symptoms look a lot alike. Get an accurate diagnosis before deciding on a control strategy. No fungicide works equally well on all diseases, and mixed infections are common on some crops.

Look out for these heat-loving diseases as temperatures begin to rise.

1. Erwinia soft rot

Soft rot diseases are caused by Erwinia carotovora and E. chrysanthemi. They attack many plants including most foliage plants, unrooted cuttings like poinsettia, achillea and osteospermum. If you produce potted callas in the South, you might see this disease in the late winter or spring.

Erwinia spp. are not host specific.

What to watch for.Erwinia causes soft or mushy rot. The bacteria actually produce enzymes that disintegrate the pectin that holds plant cells together.

Infected plants or plant parts can have a rotten, fishy odor (once you smell this, it is hard to forget). Erwinia makes this odor even in Petri dishes without any plants present.

Controls. The first line of defense is to examine new plant material and routinely rogue diseased plants. This is especially critical during propagation since the bacteria spread easily in water and thrive in the warmth and water.

If you use bactericides, be prepared for disappointment. The best products contain streptomycin sulfate, which is not legal in all states. Streptomycin sulfate was labeled as Agri-strep or Agri-Mycin originally, and some of you may still have product in stock. We have found that 8 ounces per 100 gallon rate is relatively effective.

You should alternate streptomycin sulfate with a copper product. The latter products have been somewhat less effective against soft rot diseases but can help.

Never rely on a single bactericide since resistance development can be rapid if a single active ingredient is relied on. Although Bacillus subtilis (Rhapsody) has been very good for bacterial leaf spots caused by Pseudomonas and Xanthomonas, its efficacy on Erwinia has not been good.

2. Phytophthora

Phytophthora can be present nearly year-round, especially in greenhouse crops. However, the most severe symptoms are caused in summer.

What to watch for.Phytophthora spp. tend to attack plants at the crown resulting in a rapid death. Examination of the roots can sometimes show that they are relatively unaffected and that plant death was due to stem or crown rot alone. Under cooler conditions, Phytophthora is slower to develop and looks like any other root rot -- notably Pythium.

Some of the most common plants attacked by Phytophthora are annuals (vinca, pansy and petunia), foliage plants (English ivy), potted flowering crops (poinsettia and gerbera) and woody ornamentals (azalea and gardenia).

Controls. Traditionally, the best fungicides for Phytophthora have been phosphonates (Aliette, Fosphite and Vital), mefenoxam (Subdue MAXX) and etridiazole (Terrazole and Truban). In more recent times, dimethomorph (Stature DM) has emerged as an excellent treatment. There are quite a few new active ingredients under development.

3. Cylindrocladium cutting and crown rots

Cylindrocladium spp. cause severe losses in production of a variety of important ornamentals from the tropical (spathiphyllum and Kentia palm) to the temperate (rose, myrtle and azalea). Many important diseases have been reported from the Southeastern United States, Hawaii, Australia and Mauritius indicating that Cylindrocladium spp. may be more common in warmer climates.

Cylindrocladium spp. are good saprophytes and live for long periods (often years) in the soil on organic matter. They form little survival structures called microsclerotia that withstand many extremes in the environment and simply rest until they come into contact with another susceptible plant.

What to watch for. During propagation of woody plants like azalea, myrtle and roses, the cuttings fail to root and rot from the base upward. In other plants, the crowns rot without any evident root damage. Lower leaves yellow and the entire plant may wilt. If conditions are not ideal, roots may rot without any aboveground signs of the infection at all.

Controls. Control strategies must take into account long-term soil survival of Cylindrocladium. Use new containers and growing media in propagation. If you must reuse flats, be sure to wash thoroughly (remove all organic matter) and clean in a disinfectant like bleach (or chlorine dioxide) or a quaternary ammonium.

The best fungicides have been fludioxinil (Medallion), triflumizole (Terraguard) and strobilurins like Heritage, Insignia and Compass O.

An effective treatment for cutting rot is spraying stock plants the day before taking cuttings. This allows good prevention of Cylindrocladium without delaying rooting.

Both Medallion and Terraguard have been shown to delay rooting on some woody crops. Despite this, the best-quality cuttings are often produced with a single Medallion treatment. Use rates must be determined from product labels, but our trials indicate a sprench of 4 ounces per 100 gallons of Medallion, Heritage or Terraguard yields the best results.

4. Rhizoctonia

Rhizoctonia spp. cause root, stem and foliar diseases of many of the most important container-produced ornamentals. Rhizoctonia spp. usually attack plants at the soil line, causing root loss and constriction of the stem, which results in girdling and death of the tops. The pathogen commonly causes damping-off of bedding plants, stem rot of cutting-propagated plants (like poinsettia and hydrangea) and aerial blights of woody shrubs and foliage plants (especially ferns).

In the South, Rhizoctonia can attack foliage when plants are grown close together and kept moist, decimating entire stock beds or flats in a few days when conditions are hot and wet. In other areas of the country, growers create ideal conditions for Rhizoctonia by growing plants in covered structures that are heated and irrigated overhead. Severe disease development can occur in less than a week so plants should be scouted and monitored for symptoms once a week.

What to look for. Brown, irregularly shaped spots form anywhere on foliage or stems. When humidity is high, the weblike brown mycelium of the pathogen covers infected portions of the plants. This can be seen many times in aerial blight and sometimes when flats of seedling crops are attacked.

Rhizoctonia stem rot is typified by brown cankers at the soil line. The cankers are sunken and dry in appearance and can cause plant tops to collapse.

Root rot may occur in some cases of stem rot, but not always. Root rot caused by Rhizoctonia appears much like root rots caused by other fungi with roots becoming brown, somewhat mushy and finally disintegrating.

Controls. An accurate laboratory diagnosis is crucial since fungicides that control Pythium do not control Rhizoctonia and vice-versa. Keep in mind that mixed infections (cases where two pathogens are active) are also common. Only an accurate diagnosis will allow you to choose the most complete control strategies.

Watch plants for aboveground signs of root rot, including yellow or pale-green leaves, small leaves, wilting and stunting.

Our fungicide trials over the past 15 years have identified fludioxinil (Medallion) as the best product for Rhizoctonia. Medallion rates as low as 0.5 to 1 ounce per 100 gallons are very effective.

Since I left Florida 13 years ago, I have not worked on aerial blights. so other products may be as good or better. Our trials show excellent results with thiophanate methyl (3336, Fungo and OHP6672), iprodione (Chipco 26019) and azoxystrobin (Heritage). PCNB (Terraclor) has also been used very successfully against Rhizoctonia and is probably the best product for controlling losses in the ground for bulb crops due to seasonlong activity.

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- Ann Chase

Ann Chase is president, Chase Horticultural Research Inc., (530) 620-1624; archase@chaseresearch.net; www.chaseresearchgardens.com.