Citrus Trees and Winter Leaf Drop (WLD)

[TJAnonymous]

I have a 2 yr old Meyer Lemon tree that I repotted back in August. It flourished and was doing GREAT. I was excited because I anticipate it to start actually producing fruit this next spring.... When the temps started to drop, I brought it inside to my enclosed sun room and put it in a south facing window. Then the leaves started to drop.... At first, I thought maybe it was being caused by some mite or other bug. I treated the tree with neem oil, insecticidal soap, even Sevin.....didn't stop the leaf drop. I thought maybe it wasn't getting ENOUGH light (its been pretty cloudy most days) so I took one of my chicken heat lamps and put a grow light in it, clamped it above the plant and put it on a timer. Still dropping leaves....

I was DETERMINED to figure out why.... Spent hours researching Winter Leaf Drop. In regards to my tree, most of the time, the leaves are green. Sometimes they are yellowed (chlorotic) in the center. In all cases, the leaf itself is dropping but the stem stays attached to the tree.

The key to fixing this is to understand why WLD happens. I came across this LINK on a tropical fruit forum.

We found light may NOT be the factor causing WLD, so we thought about temperature. After a long discussion with citrus experts in Florida and Israel, it was found that temperature will cause WLD. The temperature tables from the book Biology of Citrus show that citrus stops root growth and root function if the soil temperature drops below 54.5 F (12.5 C). Leaf activity will be reduced if the temperatures drops below 64 F (18 C). Leaf activity means the full process of water evaporation for leaf surface cooling, energy transformation (photosynthesis) and starch reduction for building amino acids and other compounds for forcing plant growth and cell development. Citrus controls its leaf temperature by evaporating water from the leaf blade. This reduces the temperature even during hot periods and will maintain the leaf temperature at the optimum levels between 77 F (25 C) and 95 F (35 C). But even on cold days the sunlight can heat up the leaf surface quite quickly to levels beyond the critical temperature of 54.5 F (12.5 C). Photosynthesis itself works better in cooler conditions with high light radiation than in the warmer periods of the day, so most of the photosynthetic starch production is done in the morning before noon and less water is evaporated than during the afternoon. Optimum leaf temperature for photosynthetic activity for most plants ranges from 50 F (10 C) up to 90 F (32 C). Photosynthesis itself needs carbon dioxide, light and water to transform the carbon dioxide into starch and oxygen. During the night the starch will be oxidized to provide energy needed for plant growth and development. The whole process is called breathing. Water and nutrients for the breathing process must be taken up by the roots. Oxygen and carbon dioxide will be delivered from the air around the plant, taken up by the leaf surface (and to some extent by other green parts of the plant) so leaf and root activity must run in a balance to provide the best plant performance for growth, flowering and fruit development. If a citrus tree is stored at temperatures below 54 F (12 C) but gathers enough light for photosynthesis, this balance is broken. The leaf activity requires water, which the roots cannot deliver. The tree stops evaporation and water will be unavailable for cooling the leaf surface on bright days, so the tree reduces active leaf area by leaf abscission. This seems to be the best theory about what causes WLD.

Partial or complete defoliation was never critical if the root ball was kept a little more on the dry side, but if it was too wet, a quick root decline developed even if Poncirus trifoliata was used as a root stock. Most of the trees recovered quite will in spring (if the roots stayed healthy and a heavy bloom was set). But in recovering the whole canopy, often the trees used up much of their starch reserves in the stock, which did not fully refill during the short summer times. After some years, many trees suffered, growth was stopped and the trees died because all of the starch had been depleted. So what to do about WLD? Irrigation during wintertime seems to be a recommended practice to slow down WLD. Irrigation with warm water 77-90 F (25-32 C) supports the root function, even the water uptake, so WLD will slow down. Irrigation reduces the plant stress during cold winter time and is therefore recommended.

Keeping the trees in a room with high humidity seems also to slow down WLD but cannot prevent it. Also a place more in the shade, to minimize leaf activity, slows down WLD. Keeping the root temperature below 64 F (18 C) but at or above 59 F (15 C) seems to work best for stopping WLD. The plant functions are minimized, but water and nutrient uptake for leaf activity is high enough to support the breathing process and leaf surface cooling by water evaporation. If WLD persist, force the root temperature higher, around 70 F (21 C) this should stop leaf drop. Sometimes during the winter, fruits dry out on the tree and drop if the tree is stressed too much. So for fruit development and fruit maturity, higher temperatures and good leaf activity should be maintained. Irrigation with a nutrient solution should be done even in winter.

Based upon this information, I have done 3 things:

1. Purchased a hygrometer to monitor the moisture level of the soil to ensure I am not over or under watering.
2. Purchased a warm mist humidifier.
3. Brought in ANOTHER chicken heat lamp and am using the supplemental heat in the sunroom to keep the temperature from falling below 65 degrees. I typically keep my thermostat at 70 degrees but the sunroom is a bit cooler due to all the windows. Gets LOTS of sun, but pays the price with as a cooler room due to heat loss through the wall-to-wall windows on 3 sides of the room.

Anyway, we will see if this helps solve my leaf drop. Updates to come....

 

[JCaseyHammond]

Excellent research. Soil temp below 55 degrees seems likely cause of winter leaf drop.