The 2019 growing season is underway. At this time, most greenhouses have been seeded. As the seed germinates and seedlings grow, pests, fertility and contamination due to drifting of pesticides may arise and affect the crop. Algae, Black Root Rot, Pythium Damping-off and Rhizoctonia Damping-off, are common occurrences in greenhouses. These pests are highly controlled when trays have been cleaned and steam sterilized at 80ºC (176ºF) for 60 minutes, and when muckbeds are adequately steamed at 82ºC (180ºF) for 30 minutes at a 15 cm depth. For some pests such as Algae, Black Root Rot and Pythium Damping-off in float trays, chemical control is not available, therefore, steam treatment is essential.
The following are some problems that may occur in the greenhouse during the growing season:
1- Uneven emergence
Uneven emergence (Fig. 1) may be related to seed quality, improper environmental conditions during storage of the seed, unknown genetic factors, exposure of the seed or the greenhouse environment to chemicals (i.e. un-rinsed bleach on trays), excessive algal growth on plant trays and other factors. It is a good idea to keep a count of the number of seedlings through time in several trays across the greenhouse to get an idea of how emergence is progressing.
Figure 1. Uneven seedling emergence
2-Algae
Thick mats of algae may develop on the surface of either dry or float trays, especially under cool and dim conditions (Fig. 2). A number of different types of algae may form on trays depending upon what types were in the substrate used to fill the trays or which type were present upon the trays from previous seasons. Blue-Green Algae are the most common algae in greenhouses and produce an oily-looking, blackish-green slimy layer on the tray surface. Algae will cause oxygen levels in the media to fall and inhibit root growth, as well as compete with the plants for fertilizer. In some cases Blue-Green algae seem to be toxic to the seedlings, possibly through compounds they release or by promoting the formation of nitrites in the tray media.
There are no chemical treatments for algae control in tobacco greenhouses. Algae are best managed by providing conditions as favourable as possible for seed germination and early seedling growth, so that the seedlings will establish quickly and outpace the formation of the algae on the media surface. Following good tray fertilization practices is also essential because fertilization encourages algae growth. Therefore, fertilizer should not be added to float tray water until the plants have reached the two-leaf stage. For dry trays, fertilizer applications should not start until three weeks after seeding.
Figure 2. Tobacco seedlings in trays infected with algae
3-Rhizoctonia Damping-off and Target Spot
Rhizoctonia Damping-off is caused by the soilborne fungus Rhizoctonia solani, the same fungus that causes the tobacco diseases Target Spot both in the greenhouse and field and Sore Shin in the field. R. solani also infects a large number of other cultivated crops.
With Damping-off (Fig. 3A), the fungus infects the stem at the soil line and rots the base of the plant causing plants to dry up or topple over. With Target Spot (Fig. 3B), the fungus releases spores from the soil surface that drift up onto the leaves and causes infection that appears as water-soaked, rotting lesions. Damping-off and Target Spot infected root fragments and leaves can attach to the Styrofoam and if the trays are not cleaned and steam sterilized before reusing them, these plant pieces carrying the fungus will allow it to establish in the trays and multiply, thus becoming a significant source of inoculum. In muckbeds, the infected plant parts fall onto the ground and in the presence of considerable moisture, the fungus grows and spreads contaminating more areas of the seedbed, which is often visible by patches of wilted and dead seedlings. Steam sterilization of muckbeds is an effective control for the disease.
If infection is found to be present in the greenhouse on seedlings, the disease can be controlled through applications of the registered and recommended fungicide Senator 70WP or Senator 50 SC (a.i. thiophanate-methyl 70% or 50 %).
Figure 3. A) Tobacco seedlings infected by Rhizoctonia Damping-off in muckbeds
Figure 3. B) Target Spot lesions on leaves
4- Pythium Damping-off
Pythium Damping-off in greenhouses is caused by Pythium spp. Some Pythium species produce swimming spores (zoospores) that can swim in water films on leaf surfaces, hydroponic media, float water and surface water (streams or ponds). While doing so, they can also adhere to surfaces such as roots, seeds, soil particles or tray surfaces where they can then grow and spread.
Pythium spp. infection affects the vigour of seedlings and can result in poor quality transplants. Infected seedlings show yellowing of lower leaves, stunting and wilting. The roots of infected seedlings present a light brown discolouration in the early stages of infection and later they become dark brown with a slime looking appearance. Infected roots fall off the trays leaving seedlings without a root system.
Pythium Damping-off is best controlled with steam sterilization of the Styrofoam trays. Similarly, steam sterilization also controls Pythium Damping-off in muckbed greenhouses. Pythium Damping-off can be prevented from infecting seedlings and spreading in greenhouses by not using surface water sources (e.g. ponds, creeks, etc.) to fill float beds or to irrigate seedlings, ventilating greenhouses as much as possible, keeping the trays or seedbed surface as dry as possible by the evening, promoting good drainage of the muckbed, using recommended seedling rates and avoiding high plant populations, and removing infected plants or trays from greenhouses. The fungicide Ridomil Gold 480 SL (a.i. = metalaxyl 480 g/L) is the only fungicide registered and recommended for Pythium Damping-off control, however, the fungicide cannot be used in float tray greenhouses. Ridomil Gold 480 SL must be applied within 3 days prior to seeding.
5- Black Root Rot
Black Root Rot is caused by the fungus Thielaviopsis basicola, which infects tobacco and many vegetable crops, ornamentals and field crops. Most of the Black Root Rot occurrences in tobacco reported over the years have originated in the greenhouse, particularly when trays older than five years are used.
Black Root Rot infection occurs in trays when infected roots penetrate the Styrofoam crevices and remain behind when seedlings are pulled from the trays (Fig. 4A). Also, infected root fragments can be left behind on tray surfaces. If the trays are not cleaned and steam sterilized, the fungus thrives and builds in inoculum potential from year to year and may eventually contaminate most of the trays in a greenhouse. Black Root Rot infected seedlings (Fig. 4B) have very few healthy roots and perform poorly in the field after being transplanted. These plants usually show pale green leaves and yellow discolouration of lower leaves, and will be stunted and/or wilted. Fields with Black Root Rot infected seedlings tend to be uneven throughout most of the growing season. Black Root Rot can also occur in muckbed greenhouses. There are no fungicides registered to control Black Root Rot in the greenhouse or in the field. Steam sterilization is the most reliable method to control the disease.
Figure 4. A) Infected root fragments attached to the Styrofoam (black arrows)
Figure 4. B) Tobacco seedlings with roots that are black due to Black Root Rot infection
6-Chilling injury
Chilling injury shows up as a cupping of the cotyledons and leaves (Fig. 5). Although not usually severe enough to be fatal to the seedlings, it can set them back and allow other problems, such as algae on trays, to be more damaging than they would otherwise be. To minimize chilling injury, keep an eye on greenhouse temperatures, especially in areas where cold drafts can occur. It is important to note that chilling injury can also occur after a large shift in cooler temperatures in addition to the absolute minimum temperature.
Figure 5. Cupping of cotyledons and leaves from chilling injury
7- Heat injury
Heat injury occurs when warm and sunny conditions occur during germination of tobacco seedlings. Temperatures above 37°C (100° F) in the greenhouse can severely injure or even kill transplants. This type of injury is more often seen on dry trays and dryness of the soil media exacerbates the injury. When heat injury occurs, typically the older leaves of the seedlings collapse and turn brown (Fig. 6). If the growing point of the seedlings is not damaged and is still active, seedlings may recover normally. If the growing point is damaged, plants may not recover or if so, they can produce excessive suckers. To avoid heat injury, Make sure to lower greenhouse vents/curtains once temperatures begin to approach the upper 20’s (20-25° C) (70-80°F).
Figure 6. Collapsed and brown leaves caused by heat injury
8-Phosphorus deficiency
Phosphorus deficiency is common in rapidly growing float plants where a low phosphate fertilizer has been used. Symptoms look very similar to early Target Spot and appear as small, dark green water-soaked spots on the leaf surface. Control is achieved by applying fertilizer with a higher phosphorus analysis.
9-Pavement ants
Pavement ants (also called Harvester ants) can occasionally be problems in muck bed greenhouses. These ants collect seed and seedlings from the bed and may be quite damaging by thinning down the plant stand (Fig. 7). There are no insecticides registered for Pavement ants control in muckbed greenhouses. Laying out a bait of corn meal for the ants has shown some promise as they prefer it as a food source compared to tobacco seed.
Figure 7. Thinning down of plant stand due to Pavement ants’ activity near greenhouse wall
10-Spray drift from field operations
Tobacco seedling injury in greenhouses from drifting sprays in the field is becoming more common. Being aware of what is going on in the vicinity of the greenhouse and taking the appropriate measures with vents and fans to minimize the entry of outside air when spraying operations are occurring in fields nearby, will help minimize damage caused by spray drifts.
11-Tobacco Mosaic Virus
Tobacco Mosaic Virus (TMV) is most serious when infection starts in the greenhouse because the disease is rapidly spread when clipping the plants. Transmission of the virus in this manner can lead to an entire crop being affected. Unfortunately, TMV almost never produces any visible symptoms while the seedlings are in the greenhouse, instead first being noticed after the plants have already been put out into the field. Trays from any greenhouse suspected of being contaminated with TMV should be disposed of and anything which may have come in contact with the infected plants disinfected with a strong bleach solution (i.e. diluted 1:1 with water). Particular attention must be paid to the disinfection of clippers. If in doubt, throw them out.
Always read the label before applying a pest control product!
More detailed information about Algae, Black Root Rot, Rhizoctonia and Pythium Damping-off and Tobacco Mosaic Virus can be found in this section of the CTRF website.
Post prepared and sent by Canadian Tobacco Research Foundation
Mailing Address: CTRF, P.O. Box 322, Tillsonburg, ON N4G 4H5
Telephone: 519-842-8997
Web Address: https://ctrf1.com