How are pests and diseases prevented and controlled during lemon cultivation?

The main fungal diseases of lemon fruit during storage are: black rot, sour rot, stem-end rot, blue and green mold, and anthracnose; the main physiological diseases are: dry scar and edema.

(1) Stem-end rot: divided into brown stem-end rot and black stem-end rot, their symptoms are similar, but there is no obvious boundary between the brown stem-end rot lesion and healthy tissue, and a fermented odor appears in the later stage. There is an obvious boundary between the black stem-end rot lesion and healthy tissue, and a foul odor appears in the later stage. The pathogen invades the fruit tissue from the stem end; the initial lesion is water-soaked, and the lesion continuously expands, gradually turning brown or black; rot passes through the center of the fruit faster than it develops along the peel. It occurs especially severely in rainy and humid production areas or years.

(2) Black rot: the initial symptom is blackening of the stem end, and the pathogen develops downward along the central axis of the fruit; wherever the hyphae reach, blackening and rotting occur. During fruit growth and development, the black rot pathogen is mostly present at the stem end and cannot invade the interior of the lemon tissue until the stem-end tissue is destroyed. Any conditions that weaken the fruit, such as an unsuitable environment, too late harvesting, or long-term storage, will increase the fruit's susceptibility to black rot.

(3) Green mold: refers to two storage diseases caused by Penicillium italicum and Penicillium digitatum. Their onset process and symptoms are similar. Initially the peel surface softens and water-soaked spots appear; as the lesions expand, white mycelium grows on the diseased area and quickly turns into a blue or green mold layer. From the beginning of disease onset to complete fruit rotting takes only 1-2 weeks. The pathogen spreads through conidia by contact or with airflow. It invades through wounds and cracks, so the most fundamental preventive measure is to reduce mechanical injury. In all commercial citrus production areas in the world, blue and green mold are the most common causes of postharvest rot.

(4) Sour rot: the initial symptom is slightly raised, water-soaked, light yellow to yellow spots. The pathogen mainly grows inside the fruit; in the later stage, the internal tissues of the fruit all decompose into sour, water-soaked, sticky matter. At 24-29°C, it takes only 5 days from infection to complete fruit rotting, and adjacent fruits can also be infected by contact. The pathogen invades through wounds.

(5) Anthracnose: onset begins at the stem end; the initial external symptom is gray to brown spots, after which the spots rapidly expand, sometimes extending over the entire fruit. Under humid conditions, pink spores appear on the surface, and later the spore masses become brown to black; only in the later stage does the pathogen invade deep into the fruit flesh. It mainly infects mandarin-type citrus; lemon fruit is occasionally affected, and disease severity is high in humid areas and years. The pathogen grows on dead or weak branches in the orchard and spreads from there to the fruit, entering the fruit through breaks in the peel or dead stem-end tissue.

Control methods for the above diseases: in addition to proper postharvest handling, chemical control can be used. The methods are as follows:

① Thiabendazole (Tecto, TBZ) and benomyl (benlate): are the most effective postharvest fungicides, can effectively control stem-end rot and blue and green mold, and have a certain effect on anthracnose, but are ineffective against sour rot and black rot. Both fungicides are slightly soluble in water and are often prepared as wax-water mixtures or aqueous suspensions for use; during treatment the suspension must be continuously shaken to ensure the liquid is uniform. The concentrations used are: thiabendazole 1?000mg/L, benomyl 600mg/L, with postharvest fruit dipping for 3-5 minutes.

② Sodium ortho-phenylphenate (SOPP): can control stem-end rot and blue and green mold, and also has a certain effect on sour rot, especially being more effective for oranges. A 2% sodium ortho-phenylphenate aqueous solution at pH 11.5-12.0 has the highest efficacy, so the commonly used formula is: 2% sodium ortho-phenylphenate + 0.2% NaOH + 1% hexamine, or a 1% sodium ortho-phenylphenate wax-water solution can be used.

③ Sec-butylamine: can control blue and green mold, has lower efficacy against stem-end rot, and is ineffective against sour rot and anthracnose. Sec-butylamine is a liquid with a boiling point of 63°C; it can be used as a warehouse fumigant (25-200mg/L, V/V, 42h), or prepared as a 1%-2% aqueous solution for fruit dipping treatment.

④ Biphenyl: can control stem-end rot and blue and green mold, and is ineffective against sour rot. It is a vapor-phase fungicide. The commonly used method is: impregnating permeable fruit-wrapping paper with biphenyl at a ratio of 2.35g:1 sheet (27.9cm×43.2cm) to make biphenyl fruit-wrapping paper. Biphenyl can also be dissolved in paraffin oil (or mineral oil) to make a biphenyl-paraffin mixture, which is coated on paper sheets to make biphenyl liners.

In China, preservatives commonly used in production at present also include thiophanate (1000-2000mg/L), carbendazim (800-1000mg/L), imazalil (500mg/L), sodium bicarbonate (2%), etc.

Physiological diseases

(1) Brown spot (dry scar): mostly occurs from the middle to late storage period. The initial symptom is light brown irregular spots, after which the color deepens and the lesions expand; at low temperature they develop into irregular ring-shaped lesions centered on the stem end. The oil cells at the lesions rupture, collapse, and shrink, and the affected area is limited to the outer peel. Most consider this a physiological disorder caused by low-temperature imbalance (Brooks&Mccolloch,1936; Li Peiwen,1977). When stored under low temperature and low humidity, the disease is especially severe. In storage, measures such as appropriately delaying harvest, maintaining high humidity, low O2 and high CO2, and avoiding excessively low storage temperature are used to control and reduce the incidence of brown spot.

(2) Edema disease: initially the peel becomes lighter, and irregular, translucent, water-soaked spots appear on parts of the peel; when eaten, it has a kerosene taste. After the disease becomes severe, the entire fruit becomes a translucent, water-soaked fruit, soft, easy to peel, and with a strong alcohol taste when eaten. If storage continues, it is infected by other pathogenic bacteria and rots. A large number of experiments show that excessively low storage temperature and poor ventilation causing excessive CO2 accumulation can lead to the occurrence of edema disease. As long as either of the two factors exists for a certain period, it will lead to the occurrence of edema disease; when both factors exist at the same time, the disease occurs earlier and more severely. Therefore, to prevent and control lemon fruit edema disease, attention should be paid to preventing excessively low storage temperature or excessive fluctuation, while strengthening management and paying attention to ventilation.