What are the latest control measures for wheat loose smut?

Wheat loose smut, commonly known as black smut, gray packet, etc., occurs commonly in many wheat-producing regions of China, with a general incidence rate of 1—5%, and in severe cases it can exceed 10%. In recent years, especially in areas where farmers save their own wheat seed, wheat loose smut has shown a trend of increasing year by year.

Symptoms

It mainly damages the ear, but the stem and leaves can also be affected.

1. When the ear is affected, it forms a mass of black powder covered by a layer of light gray membrane; the epidermis then ruptures, the black powder disperses, and finally a curved rachis remains. Sometimes a few healthy spikelets remain at the top of the ear while the lower part turns into black powder. In most cases, the main stem and all tillers of the diseased plant produce diseased ears, but sometimes some tillers are not affected by the pathogen and grow normally.

2. Damage to the stem is not easily seen in the field. The diseased part mostly occurs near the base of the rachis, with spore masses forming blister-like and stripe-like shapes, gray-black in color.

3. When the leaves are affected, symptoms mostly appear at the base of the leaf blade, similar to those on the stem, and occur very rarely.

After the smut fungus infects wheat, in addition to the typical external symptoms, its internal tissues also change, causing cells to enlarge, stomata to enlarge and increase in number, the palisade tissue of the mesophyll to become compact, and cells to become shorter.

Causal pathogen

The pathogen of wheat loose smut is a smut fungus. The branched hyphae of the wheat loose smut pathogen are monokaryotic; colony traits on culture medium are unstable, and hyphal growth arises only from the original promycelium cells. Chlamydospores are spherical to oval, light yellow to brown, with one half darker in color and the other half lighter, and the surface has fine spines. Spore size is 5—9 micrometers, averaging 6.5—7 micrometers. Chlamydospores can germinate in both water and nutrient solution; the minimum germination temperature is 5°C, the optimum is 20—25°C, and the maximum is 35°C.

1. Overwintering and spread

The pathogen of wheat loose smut overwinters as dormant mycelium inside the seed embryo; therefore, the only way it overwinters is through infected seed. Chlamydospores can survive in the field for only a few weeks, and after overwintering there is absolutely no possibility of infection.

⟦NL>In the field, the chlamydospores of the wheat loose smut pathogen are mainly spread by wind, entering through the openings of the glumes of the protruding or opened stamens. Generally, chlamydospores can spread to a place more than 100 m from the disease center, and at the farthest can spread to more than 1000 m away. The distance of spread is related to wind speed and air movement.

2. Infection

At present, most people believe that the infection hyphae of the wheat loose smut pathogen invade directly from the ovary wall. The flowering and pollination period is the most suitable time for pathogen infection. When the pathogen's chlamydospores land on the stigma, style, or ovary wall, they can germinate within 24 hours, grow a promycelium, and directly invade at the lower part of the ovary or at the base of the grain awn from the epidermal part of the ovary wall, then pass through the integument into the nucellus tissue, then enter the base of the grain, and lurk in the embryo. This process takes about 18 days. The pathogen can also invade from the back of the grain, entering from the seed coat, nucellus, and aleurone layer to the scutellum, and then into the meristematic tissue of the embryo.

When the seed begins to germinate, the mycelium lurking in the scutellum and growing point, along with the growth of the epicotyl, invades the first node and the crown node of the seedling, and finally enters the ear and other meristematic tissues.

3. Conditions for disease occurrence

The severity of wheat loose smut occurrence is closely related to the relative humidity during the flowering period of the previous year. After wheat heading, the field temperature is often within the suitable range required by the pathogen, so humidity becomes the dominant factor. Experiments have shown that when relative humidity is 56—85%, the incidence rate after artificial inoculation reaches 91%; under dry conditions (relative humidity 11—30%), the incidence rate is 22%.

The reason that high humidity during the flowering period leads to a higher incidence rate the following year may be that when relative humidity is high, the glumes generally open at a wider angle and remain open longer, giving the pathogen more opportunities to invade. In addition, under high humidity, chlamydospores germinate faster and the germ tubes are longer. Conversely, under drought conditions, chlamydospores germinate slowly and the germ tubes are shorter, which is unfavorable for pathogen invasion.

A gentle breeze favors spore dispersal, and foggy weather or frequent light rain favors spore germination and invasion. Heavy rain easily washes spores into the soil, causing them to lose the chance to infect, so heavy rain during the flowering period results in milder disease the following year.

Control measures

To control wheat loose smut, in addition to using resistant varieties, integrated control based mainly on planting disease-free seed and seed treatment should be adopted.

(1) Seed treatment

1. Chemical treatment: 50% Hesuian wettable powder, 25% triadimefon wettable powder, and 40% seed-dressing bifenthrin wettable powder, used at 0.2% of the seed amount for seed dressing, can all achieve good results. Mixing the above-treated seed with a coating agent and then dressing the seed can improve the control effect. When dressing seed with some chemicals, be careful not to exceed the specified amount to avoid phytotoxicity.

2. Physical disinfection

(1) Hot water seed soaking

① Variable-temperature seed soaking: first pre-soak the seed in cold water for 4—6 hours to activate the mycelium, soak in water at 49 °C for 1 minute, then in water at 52—54 for 10 minutes. This method has a relatively good control effect, but the temperature must be strictly controlled, and the operation is rather complicated, making it inconvenient to promote over large areas.

② Constant-temperature seed soaking: soak the seed in water at 44—46°C for 3 hours, then remove, cool, and dry for later use. Constant-temperature seed soaking is relatively safe and has a good control effect, can reduce the incidence rate to below 0.5%, and is convenient for large-scale treatment.

(2) Physiological sterilization treatment

Use 0.5 kg of quicklime and slaked lime, add 50 kg of clean water, and this can soak 30—50 kg of wheat seed; soak for 3—4 days at a water temperature of 20°C, 2—3 days at 25°C, and only 1 day at 35°C. After soaking, dry and sow. This method is simple to operate, easy to source materials for, and highly effective, clearly superior to hot water seed soaking. During the soaking process, the wheat seed should not be too much, with a seed thickness of 2 chi being suitable, to prevent the seed at the bottom from heating up and becoming moldy.

The mycelial effect of limewater seed soaking is a kind of physiological sterilization. Under anaerobic conditions, the seed produces acetic acid or acetaldehyde, killing the mycelium lurking inside the seed. During the treatment process, lime only plays a preservative role and has nothing to do with killing the pathogen lurking in the seed.

(2) Propagating disease-free seed

Propagate disease-free seed at good-seed farms or seed propagation bases. The seed field should be at least 100 m away from the main field, and the seed used must undergo strict treatment to ensure that all of it is disease-free. Doing a good job in propagating disease-free seed is the fundamental measure for controlling this disease and can eliminate the heavy work of seed treatment.

(3) Breeding resistant varieties

The physiological races of the wheat loose smut pathogen mutate relatively slowly; wheat resistance to loose smut is monogenic dominant inheritance, and antigen sources are also abundant, which is favorable for breeding antigen varieties. In breeding resistant varieties, one can make use of the characteristics that the pathogen cannot enter the embryo, or enters the embryo but differs in the development of meristematic tissue, as well as cleistogamous pollination disease avoidance, to carry out breeding work.

In the process of breeding new wheat varieties, in addition to targeting locally prevalent and highly destructive diseases, resistance to loose smut should also be taken as a breeding indicator for reference in the final variety selection.