Diagnosis and Integrated Control Methods for Major Diseases of Clivia

1 Diagnosis of Major Diseases of Clivia

(1) Leaf spot

Leaf spot mostly occurs on the leaves of clivia. In the early stage of the disease, the leaf tips of clivia turn yellow first, and light yellow lesions appear, with the surrounding diseased tissue appearing yellow-green. As the disease develops, the lesions continue to expand into irregular large spots, slightly sunken, light brown or gray-brown inside, with slightly raised edges that are yellow-brown. In the late stage of the disease, the leaves become withered and yellow, and the disease gradually spreads toward the root direction. On the underside of the lesions, many small black granular dots are produced. The pathogen of this disease is a fungus of the genus Phoma. The pathogen usually overwinters on diseased tissue or diseased plant residues, spreads through rain, air currents and irrigation water, and mostly infects through plant wounds. Leaf spot mostly occurs in summer. Poor air circulation, excessively high temperature and direct strong light all favor the occurrence of leaf spot.

(2) Southern blight

Southern blight occurs on clivia plants near the soil. After the disease develops, water-soaked brown soft rot appears, white silky mycelium covers the affected parts, and the mycelium spreads over the soil surface around the roots, mostly in a radiating pattern, later forming sclerotia. The sclerotia are initially white, then turn yellow, reddish-brown and finally dark; in severe cases, the base of the plant rots and withers, causing death. The pathogen of this disease is Sclerotium rolfsii. The pathogen survives the winter as mycelium and sclerotia in the surface soil layer, lives saprophytically, and often occurs in hot, humid summers. The sclerotia have strong resistance to adverse environments and can survive in the soil for 3 to 4 years.

(3) Anthracnose

Anthracnose mainly occurs on the leaves of clivia. After the disease develops, light brown or dark brown small spots appear on the leaves, gradually expanding into round, oval or semicircular lesions with concentric rings, on which black granular substances are borne; when humid, a pink liquid oozes out, which is the conidial mass of the pathogen. When anthracnose is severe, the leaves wither and fall off. The pathogen of anthracnose is Colletotrichum. This disease mostly occurs in humid, high-temperature seasons; overwatering, especially watering in the evening, and placing plants too densely all make the disease likely to occur.

(4) Soft rot

Soft rot mainly harms the pseudobulbs and leaves of clivia. In the early stage of the disease, the leaves are affected, and the heart leaves and the base of the heart leaves show dark green water-soaked lesions; the diseased leaves rot, soften and droop, spreading to the root system and making it mushy, the leaf heart falls off, and the whole plant dies. The pathogen of soft rot is Erwinia. The lesion first rots from the base of the plant, then the leaves begin to rot, and the leaves and stems rot into a mushy state with a foul odor. After 2 to 3 d, the diseased parts of the leaves show a slimy soft rot stem-rot-type lesion. There is the type in which the outer leaves are good while the heart leaves rot; there is the type in which the leaves and stem base are good while part of the leaf rots, with the entire leaf completely rotting within 12 h and gradually spreading to adjacent leaves, with the diseased part showing a water-soaked rotten-leaf-type lesion; and there is also the root-rot type with rotting roots, wilting leaves and a pus-oozing root system. The pathogen of soft rot mainly overwinters in diseased plant residues and soil. In summer, rainy and humid weather, high temperature, poor ventilation and the application of large amounts of undecomposed fertilizer all make the disease likely to occur.

(5) Root rot

Root rot occurs in clivia seeds, seedlings and mature plants. If clivia is sown under low temperature and excessive moisture, seed germination is prolonged, and after the radicle is infected by the pathogen, it changes from brown to gray-brown, becomes sunken, soft and rotten, and in some cases rots as far as the endosperm, turning the seed into an empty shell. When seedlings and mature plants are infected, the roots turn gray-brown or brown, and in the later stage become hollow with only the epidermis remaining, unable to absorb water; in severe cases, the roots all rot away. The pathogen of root rot is Fusarium. This disease can occur throughout the year; high temperature, long-term moist potting soil, excessive fertilization, root injury, and excessive watering after long-term drought can all cause root rot.

(6) Mosaic disease

Mosaic disease manifests as chlorotic stripes or yellow-green streaks of varying lengths in the leaf mesophyll of clivia, with symptoms more obvious in young leaves than in old leaves. Mosaic disease is a viral disease, generally easily transmitted when pests such as aphids, red spider mites and scale insects harm clivia.

(7) Yellow leaf disease

The causes of yellow leaf disease in clivia include: the tips of young leaves turning yellow in a water-soaked manner is due to excessive and prolonged watering; yellow tips on old leaves are caused by lack of water; yellow leaf tips and burnt roots are caused by excessive fertilization; the leaf tips turn yellow first, and new leaves turn yellow (8) Sunscald

Sunscald is a physiological disease, mostly occurring in summer and autumn, and is caused by high temperature and direct strong light shining on the leaf surface. In mild cases the leaves turn yellow; in severe cases they dehydrate, dry up and die.

2 Integrated Control of Clivia Diseases

Promptly remove diseased leaves, diseased leaf sheaths and diseased plant residues, and burn the diseased residual leaves and diseased plant bodies together to reduce infection sources and avoid the spread of pathogens. Change the soil at least once a year. The new soil must be loose and fertile to facilitate the development of the fleshy roots of clivia; at the same time, the potting soil must have good drainage and a certain water-retention capacity. When necessary, disinfect and sterilize the new soil and flower pots.

Select new clivia varieties that are disease-resistant, resistant to high temperature, tolerant of severe cold and excellent in traits. Generally speaking, variegated clivia are more susceptible to disease than green clivia; clivia with thin leaves are more susceptible to disease than those with thick leaves; clivia with glossy and waxy leaves are less susceptible to disease.

Strengthen routine management of clivia. (1) Apply more organic fertilizer, phosphate fertilizer and potassium fertilizer, appropriately control nitrogen fertilizer, so that the plants grow robustly and their disease resistance is improved. (2) Maintain good ventilation conditions, promptly add shade nets in greenhouses in summer to avoid direct strong light burning the leaves, and promptly take cooling measures such as wetting and exhaust ventilation during high-temperature periods. The temperature is best controlled at around 25°C. (3) For watering clivia, it is best to use magnetized water, rainwater, snow water and running water; the water temperature should be as close as possible to room temperature, and the pH value should preferably be 7.0 to 7.6. For young clivia plants, a spray bottle can be used to spray water directly onto the leaves. When watering large seedlings, take care to pour along the edge of the pot so as not to pour water onto the leaf surface and cause water accumulation at the leaf base, which can lead to heart rot. Generally, water once a day in spring and summer, once every other day in autumn, and once every 3 to 5 d in winter. Usually, the relative air humidity in the greenhouse is maintained at 70% to 80%, and the soil water content in the pot is preferably 20% to 40%. (4) Promptly isolate and clean up diseased clivia plants to reduce infection. For diseased plants that can still possibly be saved, remove the diseased parts, disinfect with 0.1% potassium permanganate solution for 5 to 8 minutes, then wash with water, then soak in 50 to 100 mg/L α-naphthaleneacetic acid solution for several hours, replant them in disinfected moist sandy soil or vermiculite, control the amount of watering, and after they re-root, transplant them into nutrient soil.

Use chemical methods to control clivia diseases. (1) In the early stage of disease occurrence, spray the whole clivia plant with 50% carbendazim wettable powder 1000-fold solution, 70% thiophanate-methyl wettable powder 1500-fold solution, 50% asomate·zinc·thiram wettable powder 1000-fold solution, 65% zineb wettable powder 800-fold solution or 25% methomyl wettable powder 800-fold solution, spraying once every 7 to 10 d, for 3 to 4 consecutive times, which can have a preventive effect. Use the agents alternately and spray evenly to avoid developing resistance. (2) Promptly cut off diseased clivia leaves, and apply wettable powders such as carbendazim, thiophanate-methyl and asomate·zinc·thiram to the wounds, which can effectively prevent the spread of the disease. (3) When the disease is relatively severe, use 50% carbendazim wettable powder 600-fold solution to disinfect the potting soil. (4) Control pests such as scale insects, red spider mites and snails to avoid leaf wounds and reduce infection. The method for controlling scale insects is to scrub the affected leaves with soapy water or spray them with 50% phoxim emulsifiable concentrate or 50% malathion emulsifiable concentrate 1000 to 1500-fold solution. The method for controlling red spider mites is to catch them by hand or spray with 40% omethoate emulsifiable concentrate 1000-fold solution or 40% dicofol emulsifiable concentrate 1000 to 1500-fold solution, once a week, for 2 to 3 consecutive times. The method for controlling snails is to catch them by hand or spray 50% methomyl wettable powder 3000-fold or 2.5% deltamethrin emulsifiable concentrate 2000-fold solution 1 to 2 times.