Hello Robert,

fruit set, crop load and an increased variability of fruit maturation timing are strictly dependent on climatic variables. Most of temperate deciduous species require the accumulation of chilling units to overwinter. In addition, in some circumstances, day length can be used to regulate dormancy and to overcome the winter season. The day length factor is not relevant close to the equator, because of the similar duration of night and day time. In tropical regions and for certain species the alternation of dry and humid seasons may represent the factor regulating the beginning and the end of the rest period (Borchert, 1999).

• Borchert R. (1999). Climatic periodicity, phenology, and cambium activity in tropical dry forest trees. IAWA Journal 20: 239-247.

I hope this helps,

Regards,

Alessio

Thank you, Alessio. I appreciate your efforts. I looked at your reference. I work with orchards in the American Southwest, Eastern Mojave Desert, and at 12 degrees North Latitude @ 330 m elevation in the Philippines. I am aware of the research on soil moisture triggering flowering of some tropical fruit trees/crops but I see very little information specifically on the effects of relative humidity and fruit set in temperate climate fruits. I see some on the effects on pollen tube elongation at low RH. Also some speculation on tropical tree crops. This problem seems to be reported in tropical fruits but I haven't found it reported in temperate crops. Particularly in the two crops/varieties I mentioned. I believe most fruit production is in climates that have a RH at fruit set above 30% which seems to be some sort of "magical" number for some fruit trees/species/varieties and setting fruit. I suspect this might be a "desert" problem since there aren't many orchards located in climates with extremely low RH. Even in the Sonoran and Chihuahuan deserts where orchards are more common I dont see RH reported this low during fruit set. I personally have observed 16 varieties of sweet cherries growing in the Mojave Desert which do not suffer from a lack of chilling (weather stations are located there with weather data reported every 15 seconds), flower profusely, temperatures at fruit set are 20 to 30 degrees C, ten beehives within 30 meters, 10 cm of woodchip mulch on the soil covering water basins, soil moisture levels routinely monitored, but the trees lack fruit set. Five cherries in total produced in 15 years. Yet sweet cherries of the same variety produce abundantly annually within 15 to 20 km but in areas where there is grass growing or very near a source of water. Our research facility has an irrigation pond within 150 m of these cherries. All varieties of sweet cherries and persimmon replicated five times. I would like to know if anyone else has experienced the same phenomenon under similar conditions. Our current recommendations are not to plant sweet cherries in desert regions that experience RH below 30% during flowering. But with this discrepancy between the research facility and some home yards it is difficult to support without some corroborating evidence.

Thank you, Robert Ll Morris, I think I see your point now. You might discuss the matter with scientists working with temperate crops in South Australia. As far as I know, RH reaches particularly low levels down there. In olive and peach, there is a gradient of isohydry that makes cultivars/genotypes behaving differently in response to soil water deficit and/or decreasing VPD. VPD is also affected by RH, and genotypes can have different genetic control on stomatal conductance. Perhaps, particularly low levels of RH (e.g.,

Great comments! Thank you.

I didn't more about persimmon, but in sweet cherry, i think more RH damage to the flowers due to plant pathogens as like monilia.

Thank you for commenting. But would this happen on 8 different cultivars on 40 trees for 12 years in a row?