A word from the experts: winter care for protected ornamentals

Protected ornamentals need even more attention during winter months to protect their market value against attack by common diseases, writes John Scrace.

Plant diseases are an ever-present threat for the producer of ornamental plants.  Outbreaks of fungal and bacterial diseases or viruses can have a significant impact on plant quality and profits.  Rapid diagnosis of a problem is essential if effective control measures are to be applied.  This is where Pocket Diagnostic tests can be a valuable aid to your decision-making.

During the autumn and winter months crops such as cyclamen, cineraria and poinsettia are at risk from a number of important diseases.  These can strike at any time, from newly potted plants to mature specimens at the point of marketing.  As we enter the New Year attention switches to crops such as pelargonium, fuchsia and bedding plants, but there is no respite from potential disease problems.

The risks to protected ornamentals can be divided into the categories of fungal root diseases, foliar diseases caused by fungi or bacteria, and virus problems.  In each category there are Pocket Diagnostic tests available for some of the most important problems.

Root diseases

Phytophthora, Pythium and Rhizoctonia are common causes of damping-off, root decay and stem base rotting on a wide range of ornamental plants.  Severe losses can occur with crops such as poinsettia, on which root decay is the most important disease problem. The symptoms produced by the different pathogens are often very similar.  Disease diagnosis using conventional laboratory techniques can be difficult, particularly if fungicides have been applied to the plants – these may prevent the recovery of the pathogen in laboratory tests.

Correct identification of the cause of the decay is important, as control measures will vary depending on the pathogen involved.  Whilst Phytophthora and Pythium are closely related, they are very different organisms to Rhizoctonia.  This means that fungicides developed specifically for use against Phytophthora or Pythium will have no effect if the root decay affecting your plants is caused by Rhizoctonia.

Pocket Diagnostic tests are available for all three of these pathogens.  They will give a result in minutes, and are unaffected by fungicides that may have been applied to the crop.

Foliar diseases

The most common and damaging foliar disease seen during the winter months is grey mould, caused by the fungus Botrytis cinerea.  The high humidity found in many polytunnels and glasshouses at this time of year is ideal for infection by Botrytis.  The fungus remains active at low temperatures, and can affect any ornamental crop causing a rapid decay of leaves, stems, corms or flowers.  Flower spotting due to Botrytis can also be a problem on crops such as cyclamen.

Prevention of grey mould is dependent on good hygiene, as the fungus frequently colonises damaged or senescent plant parts.  Lowering the humidity by manipulation of temperature and ventilation is also critical.  Fungicide resistance is a common problem, so it is important to have an anti-resistance strategy in place.

The Pocket Diagnostic test for Botrytis works at genus level, and in addition to grey mould could be used to test for diseases of ornamentals caused by other Botrytis species, e.g. lily disease (Botrytis elliptica), tulip fire (Botrytis tulipae).

One of the most important bacterial diseases affecting an ornamental crop is bacterial blight of Pelargonium, caused by Xanthomonas hortorum pv. pelargonii.  The leaves of plants affected by bacterial blight show symptoms of necrotic spots, or wedge-shaped areas of necrosis.  Petioles and stems may develop a dry, brown-black rot.  Propagation material can be a source of this disease, for which no chemical controls are available.  Prompt disposal of affected plants, coupled with strict hygiene, is required to counter outbreaks.

Similar symptoms can be caused on Pelargonium by the Notifiable bacterial pathogen Ralstonia solanacearum.  Pocket Diagnostic tests are available for each of these bacteria.

Viruses

The most common and damaging viruses of protected ornamental plants are Impatiens necrotic spot virus (INSV), Tomato spotted wilt virus (TSWV) and Cucumber mosaic virus (CMV).  These can be found throughout the year on protected crops, whilst CMV is also a problem on ornamentals outdoors.  All three viruses have a very wide host range and can cause a bewildering array of symptoms, including stunting, mottles, mosaics, ringspots, line patterns and flower streaking.  Some of the symptoms caused by INSV and TSWV look like those of pesticide toxicity.

As with the root disease problems described above it is important to diagnose the cause of virus symptoms, as the control measures may differ.  Destruction of affected plants is important, but so is control of the virus vector.  INSV and TSWV are transmitted by thrips (particularly the western flower thrips), whereas CMV is transmitted by a number of aphid species.

John Scrace is a freelance plant pathologist of many years’ experience working in the industry and in plant clinics.