Diseases represent a serious hazard to the production of a quality poinsettia crop. Control of poinsettia diseases should be based on disease prevention because once disease producing organisms invade plant tissue, control is much more difficult and expensive and frequently not too successful. Growers should become familiar with the common poinsettia diseases and for each disease they should have a basic understanding of:
- (I) the causal organism involved;
- (II) likely sources of pathogen introduction;
- (III) conditions required for plant infection and disease development;
- (IV) and, effective disease control measures.
(I) Causal organisms of poinsettia diseases
Bacterial and fungal pathogens are most commonly involved in damage to commercial poinsettia production in Texas. Poinsettia diseases and subsequent plant losses due to fungal pathogens cause more damage and monetary loss than do bacterial pathogens, but where bacterial diseases are severe, extensive losses can occur. Both bacteria and fungi are microscopic in most forms and because of this, it is impossible to detect their presence until plant infection and subsequent plant deterioration occurs. Therefore, it is important to anticipate potential disease problems and to modify the greenhouse environment or initiate preventive chemical control to minimize plant loss to disease.
(II) Sources of pathogen introduction
Potential poinsettia pathogens can be introduced into a greenhouse in several ways. Some pathogens, such as Botrytis sp., are commonly found in greenhouses throughout the year and cause damage throughout the year when appropriate conditions are present. Other sources of poinsettia pathogens included: a) contaminated stock plants and cuttings taken from contaminated stock plants; b) contaminated media or soil; c) contaminated water supplies, especially when lake or pond water is used; d) breakdown in general greenhouse sanitation, allowing pathogens to overseason on greenhouse benches or on infected plant debris decomposing under benches. General greenhouse sanitation is of vital importance in an overall disease control program. Growers should make sure that:
- a) hose ends are off the floor;
- b) implements or tools used in propagation and production are periodically dipped in a disinfectant;
- c) workers used a disinfectant hand dip before handling plants;
- d) cuttings dropped on the floor are discarded and tools dropped on the floor are thoroughly disinfected;
- e) pasteurize media when possible or use appropriate fungicide drenches.
REMEMBER: All other attempts at disease control will be much less effective if a sanitation program is not followed.
(III) Conditions Required for Plant Infection and Disease Development
Probably the most basic concept that growers should be aware of and should constantly keep in mind is the concept of the disease triangle, represented below:
While this diagrammatic concept may seem simplistic, a thorough understanding of its conceptual relationships can help growers prevent or minimize disease losses.
Simply stated, for plant disease to occur, three (3) factors are required. Represented as the 3 corners of the triangle, they are : 1) a susceptible host (i.e. the poinsettia plant); 2) a plant pathogen (fungus, bacterium, virus, or nematode); and 3) an appropriate environment to allow the pathogen to infect the susceptible host. If, and only if, all three of these conditions are met will plant disease occur. The elimination of any one or more corners of the disease triangle will effectively eliminate plant disease.
Elimination of the potential pathogen (corner #2) involves preventing initial introduction of the pathogen into the greenhouse or chemical protection of plant tissue. Rigid sanitation measures, as discussed earlier, are of vital importance in effectively removing corner #2 of the disease triangle. By demanding such sanitation measures, potential poinsettia diseases are eliminated or minimized.
The environment (corner #3) is an important factor in the disease triangle and one which growers can effectively use to prevent or minimize disease. Plant pathogens require specific environmental conditions to survive and to infect plant tissue. In general, stagnant, humid air resulting from poor air circulation is ideal for fungal spore formation and fungal spore germination. By simply insuring that greenhouse air circulation is adequate to prevent air stagnation and “high humidity pockets”, growers can reduce the chances of foliar diseases such as Botrytis grey mold.
Infection by several poinsettia pathogens is facilitated by plant wounds or physical injury to plants. Such wounds provide the pathogen with an ideal environmental that allows ready access to tissue that can be infected and colonized.
Infection by several root or stem rotting fungi is facilitated by excessive soil moisture. Pythium root and stem rot and Erwinia bacterial soft rot are notable examples. Environmental modification by reducing soil moisture has been shown to help reduce damage by these pathogens.
Several foliar pathogens, primarily bacterial leaf spotting pathogens, are promoted by overhead irrigation, foliar syringing or other conditions resulting in wet foliage. The accumulation of free moisture on plant tissue surfaces provides and ideal environment for infection and plant damage. Avoidance of foliar wetting can significantly reduce plant damage by both bacterial and fungal pathogens.
(IV) Disease Control Measures
Disease control in poinsettia production should always be considered as a combination of strict sanitation, environmental modification for disease reduction, and chemical control. Sanitation measures and concepts relating to the importance of environmental conditions to alleviate fungal and bacterial spread in the greenhouse have been mentioned previously. Chemical control of poinsettia diseases are frequently required for various pathogens.
The following table lists the more common poinsettia diseases that Texas growers must deal with, along with symptoms and recommended controls.
Table 1. Poinsettia Disease Problems vs. Plant Development Stage.
|Bacterial Soft Rot||X||-||-||-||-|
|Rhizoctonia Root Rot||-||X||X||-||-|
|Pythium Root Rot||-||-||-||X||X|
|Alternaria Leaf Spot||-||-||X||X||-|