Introduction to Common Citrus Varieties and Identification of Citrus Huanglongbing Symptoms

Citrus Huanglongbing (HLB) can occur and cause damage almost all year round, and symptoms can appear during every new shoot stage, but symptoms are most prominent and easiest to identify in autumn and winter, causing great harm to the branches, leaves, fruit and root system of the fruit tree. So what are the common citrus varieties? What are the symptoms of citrus Huanglongbing? Let's take a look together below.

1. What are the common citrus varieties?

1. Shatangju (sugar mandarin): goes on the market in late December, one of the stable and high-yielding citrus varieties, with a sweet-and-sour taste that is much loved.

2. Kumquat: can be marketed successively from November until May of the following year; kumquat is a late-maturing variety, and freeze protection must be paid attention to in cultivation.

3. Nanfeng Mandarin: ripens in early to mid-November each year, and has a cultivation history of more than 1,300 years so far; generally it bears fruit after three years of planting and reaches high yield in six to seven years.

4. Ponkan: also known as Lukan, ripens from mid to late November to December; an old variety whose heyday has now passed.

5. Gonggan: also called Emperor Mandarin; the official ripening period is from late November to mid-December. It has been awarded honorary titles such as "Famous Fruit of China" and "King of Chinese Mandarins" by the China Fruit Marketing Association, and has important economic value.

6. Gannan Navel Orange: ripens from late November to early December; the flesh is crisp and tender, melting in the mouth, with a rich, sweet and aromatic flavor; the peel is thick and not easy to peel.

7. Harumi: also called Papa Gan, goes on the market in mid-to-late December and can stay on the tree until April of the following year at the latest; a late-maturing hybrid citrus variety, and the most promising hybrid mandarin variety in Japan.

8. Shiranui: also called Ugly Orange, goes on the market in February to March of the following year; extremely sweet, with high risk from extreme overwintering weather and high maintenance costs, and it has alternate bearing (on and off years).

9. Wogan: goes on the market in February to March; early bearing and high yielding, widely adaptable, with beautiful fruit appearance and excellent quality; late maturing with a long harvest period, has thorns, is extremely susceptible to citrus canker, and fruit drop occurs at maturity.

10. Murcott: goes on the market from February to mid-March; early bearing and high yielding, with strong graft compatibility and strong cold resistance; late maturing, with particularly severe sunburned fruit, requiring whitewashing for sun protection, and with relatively high climate requirements.

2. What are the symptoms of citrus Huanglongbing?

1. Leaf disease symptoms

The main symptom of Huanglongbing on leaves is leaf chlorosis and yellowing. The yellowing caused by this disease generally spreads across the leaf like a yellow smoke haze and has no clear yellowing edge; in addition, the chlorosis and yellowing on the leaf generally spreads from the leaf veins, leaf margins, leaf base and other parts toward the middle mesophyll tissue, but the degree of yellowing and the size of the area vary greatly among different leaves. Some leaves are yellow-green mottled, while others turn yellow entirely; even on the same leaf, the yellow color varies from deep to light. For small leaves on diseased trees, the newly grown heart leaves mostly show symptoms of smaller leaves, pointed leaves, and yellow-green mottled mosaic.

2. Branch and shoot disease symptoms

The shoot symptoms on citrus trees with Huanglongbing are relatively prominent during every new shoot stage, among which the symptoms on summer shoots and autumn-winter shoots are the most obvious, mainly manifesting as yellowing of new shoots and failure of shoots to turn green at maturity; individual new shoots on the canopy of diseased trees change from oily green to bright yellow; in addition, the yellowing of shoots on affected diseased trees is relatively uniform and consistent, and the leaves on diseased shoots tend to drop prematurely.

3. Fruit disease symptoms

The most intuitive manifestations of Huanglongbing on citrus fruit are abnormal coloring at fruit maturity, smaller fruit size, hardened and deformed fruit, and poorer taste and surface gloss of the fruit. Common symptoms of affected citrus fruit are: (1) the fruit stalk and stem end of diseased fruit turn yellow and orange-red, but the top or other parts of the fruit remain green and do not color; (2) the size and single weight of diseased fruit are noticeably smaller than normal fruit, but the hardness of the fruit is noticeably increased; (3) the sugar content of diseased fruit is noticeably reduced, and the taste and flavor are noticeably more sour, more bland, or bitter; (4) the surface gloss and brightness of diseased fruit are noticeably reduced, and the uniformity of coloring is noticeably worse; (5) many deformed fruits such as pear-shaped fruit and barrel-shaped fruit, as well as diseased fruits with abnormal ripening and coloring such as red-nose fruit and green-head fruit, appear on diseased trees; (6) the fruit size and ripening of diseased trees are uneven, with fruits large and small, and the fruit peel is rough and thickened.

4. Flower organ disease symptoms

Affected citrus trees show obvious phenomena such as early flowering, abundant flowering and an increase in deformed flowers, and these flowers on diseased trees are difficult to set fruit later and drop in large numbers afterward.

5. Root disease symptoms

The symptoms of Huanglongbing in fruit trees mainly appear on the aboveground branches and leaves, but in cases of severe disease, the root system of diseased fruit trees shows no new roots, reduced new roots or root rot, and can even cause the death of the whole tree.

Summary of diseased tree symptoms: The most prominent symptoms of citrus Huanglongbing are yellow-green mottling and blotchy yellowing on the branches and leaves of the fruit tree, but it is necessary to distinguish Huanglongbing yellowing from yellowing due to nutrient deficiency (such as iron, zinc, magnesium, manganese deficiency, etc.), yellowing due to root diseases and pests (such as root-knot nematodes, viral disease, waterlogged and suffocated roots), fertilizer-induced yellowing, and pesticide-induced yellowing.

The above is an introduction to citrus Huanglongbing. Citrus Huanglongbing has a high incidence rate, strong transmissibility and severe harmfulness; farmers should master the above symptoms and prevent it in time to avoid unnecessary losses.

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