From the Field – Agronomy Notes: Vol. 5, Num. 3

— Written By

March 22nd-

Last week (March 15-19) brought changes in the growing conditions around the state. The two previous weeks were absolutely beautiful and brought with them a much needed dry spell. Last week was generally cool and wet (again!).

At the near onset of the cold growing conditions, I had a few communications  about suspected cold injury/shock in tobacco greenhouses. In each of the situations in question, plants ranged in size from the four-leaf stage to those that were almost ready for the first clipping. Visual symptoms have been very consistent, regardless of growth stage.

The symptoms we observed (slightly pinched leaves, yellow color in the bud, and slight upward curling of leaf margins (Figure 1)) are not necessarily a result of exposure to cool/cold air temperatures, but rather how fast that exposure took place. Remember that we had air temperatures that hit 80 degrees in the Triangle on March 12th. When the temperature is that warm outside, it’s inevitable that greenhouse curtains will be opened for cooling purposes. The rapid decline in temperature we experience as the sun begins to set is where this “shock” effect starts to take place. In fact, I’d be willing to bet that air temperature inside the greenhouse fell by 20-30 degrees in less than an hour in most places. Again, it is this sharp decline in outside temperature and the inability to maintain/increase heat inside the greenhouse that is the guilty party.

Figure 1. Typical cold injury/shock in larger tobacco seedlings. Photos provided courtesy of Mason Shaw, Edgecombe County Cooperative Extension.

Moving forward, we also need to remember that at this point the visual symptomology of cold injury/shock is completely cosmetic. With a return to warm, sunny days the plants will naturally recover. That’s the good news (the “treatment” is free!). The bad news is that the forecast for all of last week wasn’t really favorable for those conditions, and conditions this week are somewhat spotty. With that being said, I’m not at all concerned about plant recovery, but we need to be mindful that it could take a week or two for plants to look significantly better. Certainly, the plants will be growing and producers should initiate or continue to mow plants (if they’ve already started). But don’t look for a quick fix with this transient ugliness until environmental conditions become more favorable, which I hope is just around the bend.

Other Points to Consider:

-There are varietal differences in tolerance to temperature change. Hence, some varieties may show symptoms of cold injury/shock worse than others.

-Warm air distribution in the greenhouse may not be uniform, so don’t be surprised if you see “cool spots” around larger greenhouses.

-In years past, cold injury/shock was sometimes confused with boron deficiency. This was before my time, but a really nice summary of how to distinguish between the two issues was put together. Please find that Extension Article here. Experience tells us that there’s a very strong chance we’re looking at cold injury/shock and not B deficiency. Also note that cold injury/shock typically results in leaves that cup upward, whereas B deficiency usually makes leaves cup downward.
Certainly, there may be other issues taking place in tobacco greenhouses around the state, but this one is the most widespread at the current time. Please do not hesitate to reach out to your local County Agent for further guidance/information on this topic, or if you suspect something else is going wrong.