July 2012 Newsletter (vol.9) >> Keep an Eye Out
July is a month that the majority of people think about fishing, swimming or weekends at the lake. Greenhouse growers on the other hand start thinking about their winter income, poinsettias. Poinsettias are susceptible to a number of rather serious diseases, especially when the temperatures begin to rise. Since much of the country is experiencing an exceptionally hot summer, it is good to get prepared. Here is information on four hot weather poinsettia diseases.
Poinsettia scab is caused by the fungus Sphaceloma Poinsettiae. The scab pathogen causes a brown, ringed leaf spot, leaf deformity and whitish, raised blisters on the stems. These blister infections induce the production of a growth hormone, which causes infected shoots to grow taller than healthy shoots. This fungus is spread primarily by splashing water, such as during propagation or with overhead watering. In hot weather this pathogen can spread rapidly. Since treatments are preventative, it is important to recognize the disease early.
The spreading of Scab is easy to control as long as you catch it early and throw out all of the infected plants. The key is to scout carefully and often. It is also important to keep water splashing to a minimum.
2.) Botrytis Blights
Botrytis Cinerea causes one of the most common diseases of all greenhouse crops and it can be troublesome in all stages of poinsettia production. Botrytis can become serious if moderately hot weather is accompanied with high humidity. This fungus colonizes dead plant tissues which produces large numbers of spores that are easily carried from plant to plant by handling or air movement. In the early stages, tan and gray-brown blotches develop on the bracts while spot lesions appear on the leaves of mature plants. Damaged areas may become covered by a fuzzy gray growth of fungus.
In order to prevent this disease, it is key to keep the air circulating in the greenhouse. Cloudy, moist conditions provide an ideal setting for infection. It is important to never water late in the day and keep plenty of space between plants for increased air flow.
3.) Rhizoctonia Crown and Root Rot
Rhizoctonia Crown and Root Rot can be very aggressive in hot weather. Rhizoctonia can cause a disastrous root rot in a very short period of time that will eventually kill the plant. It can cause a brown, stringy rot at the soil line of recently transplanted cuttings, which resembles Botrytis crown rot. Feeding by fungus gnats greatly increases the damage.
Control this pathogen by improving greenhouse sanitation and keeping dust to a minimum. Products such as Sanidate, Zerotol and Greenshield can help you stay on top of your greenhouse sanitation. Conditions favoring disease development include planting too deeply and injuring the stem while planting.
Fungus Gnat Control: - Gnatrol
4.) Pythium Root Rot
There is a hot weather form of Pythium, which can cause extensive root rotting on poinsettias. This form of Pythium root rot causes a discoloration and decay of the small absorptive roots. The outer layer of the root is easily stripped off, leaving a narrow core tissue. Infections can extend up to the stem and result in a brown-colored canker. Rooted cuttings may be stunted and yellow with brown decayed roots. Sometimes a plant diagnostic test is needed to distinguish this root rot from the one caused by Rhizoctonia.
Using good sanitation methods will help prevent this disease as well as monitoring the water intake of the plants. It is also important to use a well-aerated growing media mix as well as to not use an excess of soluble salts (follow the label). Using too much fertilizer may result in salt burns to the roots.
Remember if you plan ahead, scout early and have good sanitation practices in place, these hot weather diseases will not come between you and your beautiful winter poinsettia crop.
“Poinsettia Diseases”. Cooperative Extension Service-University of Kentucky.
“Poinsettia Scab”. Ecke Ranch. www.ecke.com/ecke/?page_id=375