Black rot of orchid

Aug 26, 2011 7:02 AM

Causal agent: Phytophthora palmivora/ P. cactorum.

Host Range: Frequently found on Cattleya orchids and their hybrids (Brassocattleya, Laeliiocattleya etc.). Also found on other orchid genera, including Aerides, Ascocenda, Dendrobium, Gongora, Maxillaria, Miltonia, Paphiopedilum, Phalaenopsis, Rhynchostylis, Schomburghkia, and some less commonly grown genera.

Symptoms: Small black lesions can be observed on the roots or basal portions of the psuedobulbs. As the lesion age, they enlarge and may engulf the entire psuedobulb and leaf. The pathogen can spread through the rhizome to other portions of the plant, eventually killing the entire plant.

Nursery Sanitation Recommendations for Phytophthora: Fungicides should be considered as a tool for managing black rot as it can be the primary defense in an existing crop and provide at least some level of disease control. 

Management: Phytophthora cactorum and Phytophthora palmivora are considered water molds and require water to spread the spores and to germinate on new hosts. The spores can easily spread in irrigation water and can splash from one plant to another during watering. In addition, zoospores, which are motile, are normally considered the infective spore and can move readily when free water is available. Therefore, it is crucial to remove infected plants immediately to prevent further spread, reduce periods of prolonged wetness, and provide adequate ventilation. Elevating the plants above the ground or keeping them on a solid surface can also help prevent infections. 

Fungicide Options: Preventative applications may aid in reducing disease spread, but only complete control can be achieved if the infected planting material is destroyed.

Growing Media and Storage: Use only unopened bagged growing media stored on a covered paved surface that can be periodically washed down with a 1:3 ratio of bleach (sodium hypochlorite) to water. It is likely that the pathogen will move into your growing media if not bagged or completely covered. Each use, use only disinfected tools and hands (disposable latex gloves that can be purchased at the grocery store or professional cook equipment stores work well). Bleach works by oxidizing or destroying the molecular bonds in microorganisms. Store purchased bleach solutions are now usually 6% sodium hypochlorite. The older non-concentrated versions are probably around 5% solutions. Avoid mixing bleach with acids or toxic chlorine gas may result. 

Containers: Store new pots in sanitized areas similar to the growing media storage area. Your best option is to always use new potting containers, but if this is not feasible submerge potting containers in a 1:3 ratio of bleach (sodium hypochlorite) to water with agitation for a minimum of 10 minutes. 

Bench Sanitation: Make sure that bench surfaces are high enough above the soil surface to avoid splashing from the ground below. Sanitize all bench surfaces and tools used to prune or work with plants before each use. Remove or sanitize any surfaces that may drip water onto crop. Bleach dunking will cause steel to rust. Some growers handle this by dipping in bleach and then dunking in oil after drying. Examples of Disinfectants for tools and benches include: 1) 25% chlorine bleach (3 parts water and 1 part bleach; 2) 25% pine oil cleaner (3 parts water and 1 part pine oil); 3) 50% rubbing alcohol (70% isopropyl; equal parts alcohol and water); 4) 50% denatured ethanol (95%; equal parts alcohol and water); 5) 5% quaternary ammonium salts. Soak tools for 10 minutes and rinse in clean water. Do not mix quaternary ammonia with bleach. The wood portions of you bench may be very difficult to sanitize because they are porous. Scrubbing to remove algae, scum, mildew and dirt before treating may help. 

Water Supply and Hand Watering: Well and not surface water should be used unless disinfected. If hand watering is utilized, be sure to sanitize the hose and water wands with bleach solution and hang in areas where the ends of the hose or wands will not contact soil or other potentially contaminated surfaces. 

Plants Brought Into Nursery: Any new plants brought into the nursery should be kept isolated (including tools and continuous bench space used for these new plants) from other plants for at least 6 weeks to observe any disease or pest symptoms, and to avoid contamination with other crops.

Other: Check potential contamination surfaces like plant transport trailer or cart surfaces routinely.

Reference: http://edis.ifas.ufl.edu/pdffiles/PP/PP26000.pdf