Although needed in very small amounts, molybdenum (Mo) is an essential micronutrient with many functions in plants, including the assimilation of nitrogen, degradation of purines and sulfates, biosynthesis of plant hormones, etc. We already have something on molybdenum functions, absorption, requirement, sources, toxicity, etc., if you need to know more.
Molybdenum deficiency in plants will cause stunted growth, marginal and interveinal chlorosis, and pale-green or yellow-green leaves, especially in the middle leaves going upward to the plant tip. Severe cases will result in interveinal and/or marginal scorch, necrosis, or browning as nitrates accumulate in leaves.
More symptoms include leaf rolling/cupping, deformation, withering, and leaf death. Also, you should expect restricted flowering, reduced fruit setting, leaf death, and reduced yield. See more on molybdenum deficiency in plants symptoms, causes, diagnosis, etc.
All these are just generalized signs or symptoms. Specific Mo deficiency symptoms may vary from one plant to another.
Here are some of the signs to expect in various plants:
1. Corn or maize
Molybdenum deficiency in corn or maize is rare unless grown in acidic soils. Symptoms include lower leaves becoming yellow and necrotic, especially on the tip, along margins, and in-between veins. Also, leaves will have less area, and internodes will be shorter.
The other deficiency symptoms include lower dry matter yield, reduced or delayed tasseling, reduced pollen grain development, poorly developed stamen, small anthers, and low chlorophyll and Mo concentration in tissue.
2. Poinsettias
Poinsettias deficiency symptoms include a thin chlorotic band or halo on recently matured or middle leaves, leaf rolling or curling (pucker downward), and distortion. At an advanced stage, expect marginal necrosis or edge burn.
However, if the marginal yellowing occurs in older leaves, it is likely a magnesium deficiency. Also, the thin yellow band occurs from the tip to the leaf base.
3. Cucurbits
Molybdenum deficiency in cucurbits – squash, melons, gourds – will make crown leaves slightly chlorotic, i.e., have interveinal bright yellow or whitish tan color followed by brown or necrotic margins.
More signs you will notice include stunted and mishappened leaves that may have curled margins if the deficiency is severe and will progress, affecting the entire runner. Also, fruits will fail or set less, and the plant may die if not rectified.
4. Molybdenum deficiency in cannabis or weed plants
Molybdenum deficiency in cannabis plants is rare, but if it occurs, it will cause chlorosis or yellowing (including mottling or spotting), starting with leaves at the middle of the plant moving upwards.
You can confuse it with nitrogen or calcium deficiency. However, Mo deficiency will also cause a bright orange, reddish, pink, or purplish leaf discoloration on the leaf edges, moving inward. However, it can start in the middle leaf portion, not the margins, and you may also notice dark spots.
Lastly, leaves will also twist and curl or wilt. Also, they may become thick and brittle before soon falling off the plant.
5. Citrus trees and fruits
Molybdenum deficiency symptoms in citrus are large, interveinal spots in early summer, popularly known as yellow spots. As these leaves grow older, the yellow spots will have brown gum deposits on the lower sides that will later turn black.
In case of an anthracnose infection, the affected areas will fall, leaving holes. And in case of deficiency, the yellow spots will extend to the leaf margin, and leaves will drop during winter.
Citrus fruits will rarely show any Mo deficiency symptoms. However, severe cases will result in yellow spots with a brownish margin that goes through into the peel without affecting the albedo. These symptoms are common in late summer and on the sunny side of the tree.
To manage deficiency, correct soil pH by liming. Also, you can apply ammonium or sodium molybdate. However, for faster results, a 5 oz per acre for mild or 10 oz if severe will help. If used between summer and early fall, yellow spots will turn green. However, any black areas on the lower side will remain.
6. Molybdenum deficiency in tomato plant
Molybdenum deficiency in tomato plants is rare, but if it occurs, symptoms include interveinal chlorosis (mottling) on mature leaves followed by necrosis. Also, leaves may become thicker, wrinkled, mishappened, and curled (involuted), while fruit setting and yield will decline.
These symptoms resemble nitrogen, except the underside will not be reddish since a deficiency means these plants cannot utilize nitrates.
7. Cauliflower
Molybdenum deficiency in cauliflower will cause whiptail (midrib grows with minimal growth in leaf width, making the blades appear narrow or have reduced area) and marginal chlorosis followed by necrosis.
More signs include mishappened, crinkled, or even withered leaves and loose curds with small leaves or bracts penetrating them.
Lastly, Mo deficiency in cauliflower seedlings will cause severe leaf deformation, stunted growth, upward cupping, and marginal chlorosis. Also, there will be slight necrosis and reduced leaf surface.
8. Cabbage
Molybdenum deficiency in cabbage will cause pale yellow-green interveinal chlorosis or mottling, leaf cupping, and whiptail on leaves. Mature plants will have large, slightly elongated, and bent-down leaves making the plant have a loose or inhibited head formation.
On the other hand, seedlings and young plants will Mo deficiency will experience stunted growth, and leaves may turn grayish, be faint yellow-green, flaccid, or bent down. Also, these leaves may have more serrated than usual leaves.
9. Sweet potatoes
Molybdenum deficiency in sweet potatoes is rare and will cause stunted growth, mild chlorotic (pale-green) smaller leaves, and vein reddening in younger plants. Some cultivars may have silvery interveinal patches that become necrotic.
10. Okra
Levels less than 1 ppm molybdenum resulted in toxicity symptoms in okra (Hibiscus esculentus) characterized by pale yellow leaves curling upward. Upon applying 1-4 ppm Mo, plants become healthy. However, using 8-16ppm resulted in stunted growth and roots with a dark brownish coating.
11. Lettuce
Mo deficiency in lettuce causes stunted growth and leaves turning pale green, with some having transparent spots in-between veins. Leaves will begin wilting, with tips and margins followed by necrosis (browning due to tissue death). Also, the transparent areas will become necrotic and coalesce, becoming bigger.
12. Grapevines
Mo deficiency will result in Millerandage or ‘hen and chicken.’ It will cause grapevines to develop unevenly and mature bunches having fertilized underdeveloped berries. Also, there will be unfertilized swollen green ovaries.
Some cultivars may have pale green, marginal necrosis, flaccid or cupped leaves. Also, shorter, zigzag-shaped internodes may occur.
13. Wheat and oats
Signs of molybdenum deficiency in wheat and oats include chlorosis strips, necrotic regions on the middle leaf blades, white heads, shriveled and poorly developed seeds, and delayed maturity. Also, the plants will look limp.
14. Canola
Some signs of Mo deficiency in canola include stunted growth, pale green interveinal tissue, and scattered white dead spots. Severe cases will cause marginal scorching or browning. Also, the top leaves can split.
How to fix Mo deficiency?
To manage deficiency, lime to the soil to lower pH, use molybdenum fertilizers (foliar, soil, or seed treatment), and add organic matter.